tidaldb/docs/planning/milestone-3/phase-2/task-03-hard-negatives.md

# Task 03: Hard Negatives

## Context

**Milestone:** 3 -- Personalized Ranking
**Phase:** m3p2 -- Feedback Loop
**Depends On:** m3p1 Task 02 (relationship graph: `RelationshipType::Hide`, `RelationshipType::Blocks`), m3p1 Task 03 (`UserStateIndex`: `add_hide`, `add_block`), m1p2 (WAL: durability infrastructure)
**Blocks:** Task 04 (Atomic Signal Dispatch integrates hard negative writes), m3p4 (User State Filters depend on crash-safe hide/block state)
**Complexity:** L

## Objective

Deliver crash-safe hard negative storage. When a user hides an item or blocks a creator, that exclusion must be permanent and must survive process crash + WAL replay without leaking. A hidden item that reappears after restart is a trust-destroying bug.

Hard negatives have three storage layers:
1. **WAL event**: durable, append-only, replayed on crash recovery
2. **Relationship edge**: permanent storage in users keyspace (`RelationshipType::Hide` or `RelationshipType::Blocks`)
3. **In-memory bitmap**: `UserStateIndex.add_hide()` / `UserStateIndex.add_block()` for O(1) query-time filtering

This task delivers the WAL event format for relationship changes, the replay logic that reconstructs hide/block state from WAL events, and the `HardNegativeStore` that coordinates all three layers.

The critical invariant: **for any sequence of hide/block/signal events, a RETRIEVE query NEVER returns a hidden item or a blocked creator's items**. This invariant is enforced by a property test that generates random event sequences and verifies exclusion.

## Requirements

- `WalRelationshipEvent` struct: WAL event for relationship changes (hide, block, unblock)
- WAL event format extends existing WAL wire format with a `RelationshipChange` tag
- `HardNegativeStore` struct: coordinates WAL write, storage write, and bitmap update
- `store.hide_item(user_id, item_id, timestamp)` atomically: appends to WAL, writes Hide edge, adds to `UserStateIndex`
- `store.block_creator(user_id, creator_id, timestamp)` atomically: appends to WAL, writes Blocks edge, adds to `UserStateIndex`
- `store.replay_wal_event(event)` replays a WAL relationship event into storage + bitmap
- Hide events point from user to item (`RelationshipType::Hide`)
- Block events point from user to creator (`RelationshipType::Blocks`)
- Hide and block are permanent -- no automatic expiry
- WAL replay reconstructs all hide/block state correctly
- Crash at any point during the hide/block write path never produces a state where the hide/block is lost
- `unblock_creator(user_id, creator_id)` and `unhide_item(user_id, item_id)` for explicit reversal

## Technical Design

### Module Structure

```
tidal/src/
  entities/
    hard_neg.rs -- HardNegativeStore, WalRelationshipEvent, replay
```

### WAL Event Extension

```rust
// === entities/hard_neg.rs ===

use crate::schema::{EntityId, Timestamp};
use crate::entities::relationship::RelationshipType;

/// WAL event for a relationship change.
///
/// Extends the WAL event format to support durable relationship mutations.
/// The WAL is the source of truth for crash recovery: on restart, all
/// relationship change events are replayed to rebuild in-memory state.
#[derive(Debug, Clone, PartialEq)]
pub struct WalRelationshipEvent {
    /// User performing the action.
    pub user_id: EntityId,
    /// Target entity (item for hide, creator for block).
    pub target_id: EntityId,
    /// Relationship type (Hide or Blocks).
    pub rel_type: RelationshipType,
    /// Whether this is an add (true) or remove (false) operation.
    pub is_add: bool,
    /// Timestamp of the event.
    pub timestamp_nanos: u64,
}

/// Wire format for WAL relationship events.
///
/// ```text
/// [tag: 1 byte = 0x52 ('R' for Relationship)]
/// [user_id: 8 bytes BE]
/// [target_id: 8 bytes BE]
/// [rel_type: 1 byte]
/// [is_add: 1 byte (0 or 1)]
/// [timestamp: 8 bytes LE]
/// ```
///
/// Total: 27 bytes (fixed size).
pub const WAL_REL_TAG: u8 = 0x52; // 'R'

pub fn serialize_wal_rel_event(event: &WalRelationshipEvent) -> [u8; 27] {
    let mut buf = [0u8; 27];
    buf[0] = WAL_REL_TAG;
    buf[1..9].copy_from_slice(&event.user_id.to_be_bytes());
    buf[9..17].copy_from_slice(&event.target_id.to_be_bytes());
    buf[17] = event.rel_type.as_byte();
    buf[18] = if event.is_add { 1 } else { 0 };
    buf[19..27].copy_from_slice(&event.timestamp_nanos.to_le_bytes());
    buf
}

pub fn deserialize_wal_rel_event(buf: &[u8]) -> Option<WalRelationshipEvent> {
    if buf.len() < 27 || buf[0] != WAL_REL_TAG {
        return None;
    }
    let user_id = EntityId::new(u64::from_be_bytes(buf[1..9].try_into().ok()?));
    let target_id = EntityId::new(u64::from_be_bytes(buf[9..17].try_into().ok()?));
    let rel_type = RelationshipType::from_byte(buf[17])?;
    let is_add = buf[18] != 0;
    let timestamp_nanos = u64::from_le_bytes(buf[19..27].try_into().ok()?);

    Some(WalRelationshipEvent {
        user_id,
        target_id,
        rel_type,
        is_add,
        timestamp_nanos,
    })
}
```

### HardNegativeStore

```rust
use crate::entities::user_state::UserStateIndex;
use crate::storage::StorageEngine;

/// Coordinates hard negative writes across WAL, storage, and in-memory bitmap.
///
/// The write order is:
/// 1. Append to WAL (durability first)
/// 2. Write relationship edge to storage (persistent state)
/// 3. Update in-memory bitmap (query-time filter)
///
/// If the process crashes after step 1 but before step 2/3, WAL replay
/// on the next startup will re-execute steps 2 and 3 from the WAL event.
/// This is safe because relationship writes are idempotent.
pub struct HardNegativeStore {
    /// Reference to the user state index for in-memory bitmap updates.
    /// The store borrows this; ownership remains with TidalDb.
    _phantom: std::marker::PhantomData<()>,
}

impl HardNegativeStore {
    /// Hide an item for a user.
    ///
    /// Atomically:
    /// 1. Append WAL relationship event
    /// 2. Write Hide edge to users keyspace
    /// 3. Add item to UserStateIndex.hidden_items
    ///
    /// # Ordering guarantee
    ///
    /// After this method returns, the item is excluded from all
    /// RETRIEVE queries for this user, even if the process crashes
    /// immediately after (WAL replay will re-apply).
    pub fn hide_item(
        user_id: EntityId,
        item_id: EntityId,
        timestamp: Timestamp,
        wal_writer: &dyn WalRelWriter,
        storage: &dyn StorageEngine,
        user_state: &UserStateIndex,
    ) -> crate::Result<()> {
        let event = WalRelationshipEvent {
            user_id,
            target_id: item_id,
            rel_type: RelationshipType::Hide,
            is_add: true,
            timestamp_nanos: timestamp.as_nanos(),
        };

        // 1. WAL first.
        wal_writer.append_relationship(&event)?;

        // 2. Persist to storage.
        Self::write_edge_to_storage(storage, &event)?;

        // 3. Update in-memory bitmap.
        user_state.add_hide(user_id, item_id);

        Ok(())
    }

    /// Block a creator for a user.
    ///
    /// Atomically:
    /// 1. Append WAL relationship event
    /// 2. Write Blocks edge to users keyspace
    /// 3. Add creator to UserStateIndex.blocked_creators
    pub fn block_creator(
        user_id: EntityId,
        creator_id: EntityId,
        timestamp: Timestamp,
        wal_writer: &dyn WalRelWriter,
        storage: &dyn StorageEngine,
        user_state: &UserStateIndex,
    ) -> crate::Result<()> {
        let event = WalRelationshipEvent {
            user_id,
            target_id: creator_id,
            rel_type: RelationshipType::Blocks,
            is_add: true,
            timestamp_nanos: timestamp.as_nanos(),
        };

        // 1. WAL first.
        wal_writer.append_relationship(&event)?;

        // 2. Persist to storage.
        Self::write_edge_to_storage(storage, &event)?;

        // 3. Update in-memory bitmap.
        user_state.add_block(user_id, creator_id);

        Ok(())
    }

    /// Unhide an item (explicit reversal).
    pub fn unhide_item(
        user_id: EntityId,
        item_id: EntityId,
        timestamp: Timestamp,
        wal_writer: &dyn WalRelWriter,
        storage: &dyn StorageEngine,
        user_state: &UserStateIndex,
    ) -> crate::Result<()> {
        let event = WalRelationshipEvent {
            user_id,
            target_id: item_id,
            rel_type: RelationshipType::Hide,
            is_add: false,
            timestamp_nanos: timestamp.as_nanos(),
        };

        wal_writer.append_relationship(&event)?;
        Self::delete_edge_from_storage(storage, &event)?;
        user_state.remove_hide(user_id, item_id);

        Ok(())
    }

    /// Unblock a creator (explicit reversal).
    pub fn unblock_creator(
        user_id: EntityId,
        creator_id: EntityId,
        timestamp: Timestamp,
        wal_writer: &dyn WalRelWriter,
        storage: &dyn StorageEngine,
        user_state: &UserStateIndex,
    ) -> crate::Result<()> {
        let event = WalRelationshipEvent {
            user_id,
            target_id: creator_id,
            rel_type: RelationshipType::Blocks,
            is_add: false,
            timestamp_nanos: timestamp.as_nanos(),
        };

        wal_writer.append_relationship(&event)?;
        Self::delete_edge_from_storage(storage, &event)?;
        user_state.remove_block(user_id, creator_id);

        Ok(())
    }

    /// Replay a WAL relationship event during crash recovery.
    ///
    /// Applies steps 2 (storage write) and 3 (bitmap update) from the
    /// WAL event. This is idempotent: replaying the same event twice
    /// produces the same state.
    pub fn replay_wal_event(
        event: &WalRelationshipEvent,
        storage: &dyn StorageEngine,
        user_state: &UserStateIndex,
    ) -> crate::Result<()> {
        if event.is_add {
            Self::write_edge_to_storage(storage, event)?;
            match event.rel_type {
                RelationshipType::Hide => {
                    user_state.add_hide(event.user_id, event.target_id);
                }
                RelationshipType::Blocks => {
                    user_state.add_block(event.user_id, event.target_id);
                }
                _ => {} // Other types handled elsewhere
            }
        } else {
            Self::delete_edge_from_storage(storage, event)?;
            match event.rel_type {
                RelationshipType::Hide => {
                    user_state.remove_hide(event.user_id, event.target_id);
                }
                RelationshipType::Blocks => {
                    user_state.remove_block(event.user_id, event.target_id);
                }
                _ => {}
            }
        }
        Ok(())
    }

    /// Write a relationship edge to storage.
    fn write_edge_to_storage(
        storage: &dyn StorageEngine,
        event: &WalRelationshipEvent,
    ) -> crate::Result<()> {
        use crate::entities::relationship::{
            encode_relationship_key, encode_relationship_value,
        };
        let key = encode_relationship_key(event.user_id, event.rel_type, event.target_id);
        let value = encode_relationship_value(1.0, event.timestamp_nanos);
        storage.put(&key, &value).map_err(crate::LumenError::from)
    }

    /// Delete a relationship edge from storage.
    fn delete_edge_from_storage(
        storage: &dyn StorageEngine,
        event: &WalRelationshipEvent,
    ) -> crate::Result<()> {
        use crate::entities::relationship::encode_relationship_key;
        let key = encode_relationship_key(event.user_id, event.rel_type, event.target_id);
        storage.delete(&key).map_err(crate::LumenError::from)
    }
}

/// Trait for appending relationship events to the WAL.
///
/// Separated from `WalWriter` (signal events) because the event format
/// and tag byte are different. The `WalHandleWriter` implements both traits.
pub trait WalRelWriter: Send + Sync {
    fn append_relationship(&self, event: &WalRelationshipEvent) -> crate::Result<()>;
}

/// No-op WAL writer for testing.
pub struct NoopWalRelWriter;

impl WalRelWriter for NoopWalRelWriter {
    fn append_relationship(&self, _event: &WalRelationshipEvent) -> crate::Result<()> {
        Ok(())
    }
}
```

## Test Strategy

### Unit Tests

```rust
#[test]
fn wal_rel_event_serialize_roundtrip() {
    let event = WalRelationshipEvent {
        user_id: EntityId::new(42),
        target_id: EntityId::new(999),
        rel_type: RelationshipType::Hide,
        is_add: true,
        timestamp_nanos: 1_000_000_000_000_000_000,
    };
    let bytes = serialize_wal_rel_event(&event);
    let recovered = deserialize_wal_rel_event(&bytes).unwrap();
    assert_eq!(recovered, event);
}

#[test]
fn wal_rel_event_serialize_block() {
    let event = WalRelationshipEvent {
        user_id: EntityId::new(1),
        target_id: EntityId::new(77),
        rel_type: RelationshipType::Blocks,
        is_add: true,
        timestamp_nanos: 2_000_000_000_000_000_000,
    };
    let bytes = serialize_wal_rel_event(&event);
    assert_eq!(bytes.len(), 27);
    let recovered = deserialize_wal_rel_event(&bytes).unwrap();
    assert_eq!(recovered.rel_type, RelationshipType::Blocks);
    assert!(recovered.is_add);
}

#[test]
fn wal_rel_event_remove() {
    let event = WalRelationshipEvent {
        user_id: EntityId::new(1),
        target_id: EntityId::new(42),
        rel_type: RelationshipType::Hide,
        is_add: false,
        timestamp_nanos: 1_000_000_000_000_000_000,
    };
    let bytes = serialize_wal_rel_event(&event);
    let recovered = deserialize_wal_rel_event(&bytes).unwrap();
    assert!(!recovered.is_add);
}

#[test]
fn hide_item_updates_all_layers() {
    let user_state = UserStateIndex::new();
    let storage = InMemoryBackend::default();
    let wal = NoopWalRelWriter;

    HardNegativeStore::hide_item(
        EntityId::new(1), EntityId::new(42),
        Timestamp::now(), &wal, &storage, &user_state,
    ).unwrap();

    // In-memory: hidden
    assert!(user_state.is_hidden(EntityId::new(1), EntityId::new(42)));

    // Storage: edge exists
    let key = encode_relationship_key(
        EntityId::new(1), RelationshipType::Hide, EntityId::new(42),
    );
    assert!(storage.get(&key).unwrap().is_some());
}

#[test]
fn block_creator_updates_all_layers() {
    let user_state = UserStateIndex::new();
    let storage = InMemoryBackend::default();
    let wal = NoopWalRelWriter;

    HardNegativeStore::block_creator(
        EntityId::new(1), EntityId::new(77),
        Timestamp::now(), &wal, &storage, &user_state,
    ).unwrap();

    assert!(user_state.is_blocked(EntityId::new(1), EntityId::new(77)));

    let key = encode_relationship_key(
        EntityId::new(1), RelationshipType::Blocks, EntityId::new(77),
    );
    assert!(storage.get(&key).unwrap().is_some());
}

#[test]
fn unhide_item_removes_from_all_layers() {
    let user_state = UserStateIndex::new();
    let storage = InMemoryBackend::default();
    let wal = NoopWalRelWriter;

    // Hide then unhide.
    HardNegativeStore::hide_item(
        EntityId::new(1), EntityId::new(42),
        Timestamp::now(), &wal, &storage, &user_state,
    ).unwrap();
    assert!(user_state.is_hidden(EntityId::new(1), EntityId::new(42)));

    HardNegativeStore::unhide_item(
        EntityId::new(1), EntityId::new(42),
        Timestamp::now(), &wal, &storage, &user_state,
    ).unwrap();
    assert!(!user_state.is_hidden(EntityId::new(1), EntityId::new(42)));

    let key = encode_relationship_key(
        EntityId::new(1), RelationshipType::Hide, EntityId::new(42),
    );
    assert!(storage.get(&key).unwrap().is_none());
}

#[test]
fn replay_wal_event_reconstructs_state() {
    let user_state = UserStateIndex::new();
    let storage = InMemoryBackend::default();

    let event = WalRelationshipEvent {
        user_id: EntityId::new(1),
        target_id: EntityId::new(42),
        rel_type: RelationshipType::Hide,
        is_add: true,
        timestamp_nanos: 1_000_000_000_000_000_000,
    };

    HardNegativeStore::replay_wal_event(&event, &storage, &user_state).unwrap();

    assert!(user_state.is_hidden(EntityId::new(1), EntityId::new(42)));
    let key = encode_relationship_key(
        EntityId::new(1), RelationshipType::Hide, EntityId::new(42),
    );
    assert!(storage.get(&key).unwrap().is_some());
}

#[test]
fn replay_is_idempotent() {
    let user_state = UserStateIndex::new();
    let storage = InMemoryBackend::default();

    let event = WalRelationshipEvent {
        user_id: EntityId::new(1),
        target_id: EntityId::new(42),
        rel_type: RelationshipType::Hide,
        is_add: true,
        timestamp_nanos: 1_000_000_000_000_000_000,
    };

    // Replay same event twice.
    HardNegativeStore::replay_wal_event(&event, &storage, &user_state).unwrap();
    HardNegativeStore::replay_wal_event(&event, &storage, &user_state).unwrap();

    // State should be the same as replaying once.
    assert!(user_state.is_hidden(EntityId::new(1), EntityId::new(42)));
}

#[test]
fn replay_remove_after_add() {
    let user_state = UserStateIndex::new();
    let storage = InMemoryBackend::default();

    let add = WalRelationshipEvent {
        user_id: EntityId::new(1),
        target_id: EntityId::new(42),
        rel_type: RelationshipType::Hide,
        is_add: true,
        timestamp_nanos: 1_000_000_000_000_000_000,
    };
    let remove = WalRelationshipEvent {
        user_id: EntityId::new(1),
        target_id: EntityId::new(42),
        rel_type: RelationshipType::Hide,
        is_add: false,
        timestamp_nanos: 2_000_000_000_000_000_000,
    };

    HardNegativeStore::replay_wal_event(&add, &storage, &user_state).unwrap();
    HardNegativeStore::replay_wal_event(&remove, &storage, &user_state).unwrap();

    assert!(!user_state.is_hidden(EntityId::new(1), EntityId::new(42)));
}
```

### Property Tests

```rust
use proptest::prelude::*;

/// Represents a user action in a random event sequence.
#[derive(Debug, Clone)]
enum UserAction {
    Hide(u64),    // item_id
    Unhide(u64),  // item_id
    Block(u64),   // creator_id
    Unblock(u64), // creator_id
}

fn action_strategy() -> impl Strategy<Value = UserAction> {
    prop_oneof![
        (1u64..100).prop_map(UserAction::Hide),
        (1u64..100).prop_map(UserAction::Unhide),
        (1u64..50).prop_map(UserAction::Block),
        (1u64..50).prop_map(UserAction::Unblock),
    ]
}

proptest! {
    #[test]
    fn hidden_items_never_leak(
        actions in proptest::collection::vec(action_strategy(), 1..100),
    ) {
        let user_state = UserStateIndex::new();
        let storage = InMemoryBackend::default();
        let wal = NoopWalRelWriter;
        let user = EntityId::new(1);
        let mut ts_ns = 1_000_000_000_000_000_000u64;

        for action in &actions {
            let ts = Timestamp::from_nanos(ts_ns);
            match action {
                UserAction::Hide(item_id) => {
                    let _ = HardNegativeStore::hide_item(
                        user, EntityId::new(*item_id), ts, &wal, &storage, &user_state,
                    );
                }
                UserAction::Unhide(item_id) => {
                    let _ = HardNegativeStore::unhide_item(
                        user, EntityId::new(*item_id), ts, &wal, &storage, &user_state,
                    );
                }
                UserAction::Block(creator_id) => {
                    let _ = HardNegativeStore::block_creator(
                        user, EntityId::new(*creator_id), ts, &wal, &storage, &user_state,
                    );
                }
                UserAction::Unblock(creator_id) => {
                    let _ = HardNegativeStore::unblock_creator(
                        user, EntityId::new(*creator_id), ts, &wal, &storage, &user_state,
                    );
                }
            }
            ts_ns += 1_000_000_000;
        }

        // Verify: for every hidden item, is_hidden returns true.
        // For every blocked creator, is_blocked returns true.
        // This is the consistency invariant.
        for action in &actions {
            match action {
                UserAction::Hide(item_id) => {
                    // Check if it was subsequently unhidden.
                    let was_unhidden = actions.iter().any(|a| matches!(a, UserAction::Unhide(id) if id == item_id));
                    // The final state depends on the last action.
                    // We verify against the in-memory state.
                    let hidden = user_state.is_hidden(user, EntityId::new(*item_id));
                    // Storage should agree with in-memory.
                    let key = encode_relationship_key(
                        user, RelationshipType::Hide, EntityId::new(*item_id),
                    );
                    let in_storage = storage.get(&key).unwrap().is_some();
                    prop_assert_eq!(hidden, in_storage,
                        "memory/storage disagree for hidden item {}", item_id);
                }
                UserAction::Block(creator_id) => {
                    let blocked = user_state.is_blocked(user, EntityId::new(*creator_id));
                    let key = encode_relationship_key(
                        user, RelationshipType::Blocks, EntityId::new(*creator_id),
                    );
                    let in_storage = storage.get(&key).unwrap().is_some();
                    prop_assert_eq!(blocked, in_storage,
                        "memory/storage disagree for blocked creator {}", creator_id);
                }
                _ => {}
            }
        }
    }

    #[test]
    fn wal_rel_event_roundtrip(
        user_id in 1u64..10000,
        target_id in 1u64..10000,
        type_byte in 2u8..=5u8, // Blocks=2, InteractionWeight=3, Hide=4, Mute=5
        is_add in proptest::bool::ANY,
        ts in 0u64..u64::MAX,
    ) {
        if let Some(rel_type) = RelationshipType::from_byte(type_byte) {
            let event = WalRelationshipEvent {
                user_id: EntityId::new(user_id),
                target_id: EntityId::new(target_id),
                rel_type,
                is_add,
                timestamp_nanos: ts,
            };
            let bytes = serialize_wal_rel_event(&event);
            let recovered = deserialize_wal_rel_event(&bytes);
            prop_assert!(recovered.is_some());
            prop_assert_eq!(recovered.unwrap(), event);
        }
    }
}
```

## Acceptance Criteria

- [ ] `WalRelationshipEvent` struct with user_id, target_id, rel_type, is_add, timestamp
- [ ] `serialize_wal_rel_event` produces fixed 27-byte wire format
- [ ] `deserialize_wal_rel_event` roundtrips correctly (property tested)
- [ ] `HardNegativeStore::hide_item` writes to WAL, storage, and in-memory bitmap
- [ ] `HardNegativeStore::block_creator` writes to WAL, storage, and in-memory bitmap
- [ ] `HardNegativeStore::unhide_item` removes from all three layers
- [ ] `HardNegativeStore::unblock_creator` removes from all three layers
- [ ] `replay_wal_event` reconstructs state from WAL events
- [ ] Replay is idempotent (same event replayed twice = same state)
- [ ] Replay of add followed by remove produces correct final state
- [ ] Property test: memory and storage always agree on hide/block state
- [ ] Property test: WAL event serialize/deserialize roundtrip
- [ ] `remove_hide` and `remove_block` methods added to `UserStateIndex` (extends m3p1 Task 03)
- [ ] `cargo clippy -- -D warnings` passes
- [ ] All tests pass

## Research References

- [docs/research/tidaldb_signal_ledger.md](../../../research/tidaldb_signal_ledger.md) -- WAL-first durability pattern
- [thoughts.md](../../../../thoughts.md) -- Part V.5-6 (quarantine-first, WAL convergence)

## Implementation Notes

- The `WalRelWriter` trait is separate from the existing `WalWriter` trait (signal events). This is because the wire format is different: signal events use a signal-specific format with signal_type_id, entity_id, weight, timestamp; relationship events use the `WalRelationshipEvent` format. The `WalHandleWriter` from `db/wal_bridge.rs` will implement both traits.
- The `WAL_REL_TAG` byte (0x52) must not collide with the existing WAL event tags. Check the WAL module's tag bytes before implementation.
- The `HardNegativeStore` is stateless (no fields) -- it operates on references passed to each method. This is intentional: the state lives in `UserStateIndex` (in-memory) and the storage engine (persistent). The store is a coordinator, not an owner.
- `remove_hide` and `remove_block` are not implemented in m3p1 Task 03. This task must add them to `UserStateIndex`. For `remove_hide`: remove the item_id from `hidden_items` bitmap. For `remove_block`: remove the creator_id from `blocked_creators` set.
- The write order (WAL -> storage -> bitmap) is critical. If the process crashes after WAL write but before storage/bitmap update, WAL replay will complete the operation. If the process crashes after storage write but before bitmap update, `UserStateIndex::rebuild_from_relationships` (m3p1 Task 03) will rebuild the bitmap from storage on next startup.
- Hide events use `weight: 1.0` in the relationship edge (the weight is meaningless for boolean relationships). This is fine because the relationship type is what matters, not the weight.
- Do NOT implement the integration with `db.signal()` in this task. The wiring that routes "hide" and "block" signal types to `HardNegativeStore` instead of the signal ledger is done in Task 04 (Atomic Signal Dispatch).