# Task 01: TenantId + TenantConfig

## Delivers

`TenantId(u64)` and `TenantConfig` in `tidal/src/replication/tenant.rs`. Per-tenant quotas (signals/sec token bucket, max entities, max storage bytes). WAL segment directories namespaced under `{data_dir}/tenants/{tenant_id}/wal/`. `TidalError::QuotaExceeded` returned when limits are breached.

## Complexity: M

## Dependencies

- Phase 8.1 (ShardId, RegionId)
- Phase 8.2 (WAL segment naming)

## Technical Design

```rust
// tidal/src/replication/tenant.rs

/// Tenant identity type.
///
/// A tenant is an agent workspace or an isolated application namespace.
/// All data (WAL segments, signal ledger state, entity metadata) is
/// scoped to a tenant's filesystem directory.
///
/// `TenantId(0)` is the default single-tenant ID used by non-multi-tenant
/// deployments. This ensures backward compatibility with all existing code.
#[derive(
    Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash,
    Default,
    serde::Serialize, serde::Deserialize,
)]
pub struct TenantId(pub u64);

impl TenantId {
    /// The default tenant ID for single-tenant deployments.
    pub const DEFAULT: Self = Self(0);
}

impl std::fmt::Display for TenantId {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "t{}", self.0)
    }
}

/// Per-tenant resource configuration.
///
/// Enforced at write time. Violations return `TidalError::QuotaExceeded`.
#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
pub struct TenantConfig {
    pub tenant_id: TenantId,

    /// Maximum signals per second (token bucket rate limit).
    ///
    /// `None` means unlimited (trusted internal tenant).
    pub max_signals_per_sec: Option<u32>,

    /// Maximum number of distinct entities (items + users + creators).
    ///
    /// Checked on entity create; `None` means unlimited.
    pub max_entities: Option<u64>,

    /// Maximum total storage in bytes for this tenant's data directory.
    ///
    /// Checked on WAL segment seal; `None` means unlimited.
    pub max_storage_bytes: Option<u64>,

    /// Residency policy: which regions this tenant's data must reside in.
    ///
    /// Empty = no restriction. Used by `TenantRouter` to constrain placement.
    pub required_regions: Vec<RegionId>,

    /// Human-readable label for this tenant (for monitoring/logging).
    pub label: String,
}

impl TenantConfig {
    /// Default config: unlimited quotas, no residency constraint.
    pub fn default_tenant() -> Self {
        Self {
            tenant_id: TenantId::DEFAULT,
            max_signals_per_sec: None,
            max_entities: None,
            max_storage_bytes: None,
            required_regions: Vec::new(),
            label: "default".to_string(),
        }
    }
}

/// Token bucket rate limiter for per-tenant signal ingestion.
///
/// Refills at `max_signals_per_sec` tokens per second.
/// Costs 1 token per signal write. Bucket max = 2x rate (burst headroom).
#[derive(Debug)]
pub struct TenantRateLimiter {
    /// Current tokens (f64 for sub-token precision).
    tokens: AtomicF64,
    /// Refill rate (tokens/ns).
    refill_rate_per_ns: f64,
    /// Maximum bucket size (tokens).
    max_tokens: f64,
    /// Last refill timestamp (ns).
    last_refill_ns: AtomicU64,
}

impl TenantRateLimiter {
    pub fn new(max_signals_per_sec: u32) -> Self {
        let rate_per_ns = max_signals_per_sec as f64 / 1_000_000_000.0;
        let max_tokens = (max_signals_per_sec as f64) * 2.0; // 2s burst
        Self {
            tokens: AtomicF64::new(max_tokens),
            refill_rate_per_ns: rate_per_ns,
            max_tokens,
            last_refill_ns: AtomicU64::new(crate::util::now_ns()),
        }
    }

    /// Try to consume 1 token. Returns `Ok(())` if allowed, `Err(QuotaExceeded)` if throttled.
    pub fn try_acquire(&self) -> Result<()> {
        let now = crate::util::now_ns();
        let last = self.last_refill_ns.load(Ordering::Relaxed);
        let elapsed_ns = now.saturating_sub(last);

        let refill = elapsed_ns as f64 * self.refill_rate_per_ns;
        let new_tokens = (self.tokens.load(Ordering::Relaxed) + refill)
            .min(self.max_tokens);

        if new_tokens < 1.0 {
            return Err(TidalError::QuotaExceeded("signal rate limit exceeded".into()));
        }

        self.tokens.store(new_tokens - 1.0, Ordering::Relaxed);
        self.last_refill_ns.store(now, Ordering::Relaxed);
        Ok(())
    }
}
```

### WAL Directory Namespacing

```rust
// tidal/src/wal/segment.rs (additions)

/// Build the tenant-scoped WAL directory path.
///
/// For `TenantId::DEFAULT` (backward compat): returns `{data_dir}/wal/` unchanged.
/// For other tenants: returns `{data_dir}/tenants/{tenant_id}/wal/`.
pub fn tenant_wal_dir(data_dir: &Path, tenant_id: TenantId) -> PathBuf {
    if tenant_id == TenantId::DEFAULT {
        data_dir.join("wal")
    } else {
        data_dir
            .join("tenants")
            .join(tenant_id.0.to_string())
            .join("wal")
    }
}
```

## Acceptance Criteria

- [ ] `TenantId` is `Copy + Clone + Debug + Eq + Hash + Ord + Default + Serialize + Deserialize`
- [ ] `TenantId::DEFAULT` is `TenantId(0)`; all existing code using `TenantId(0)` works unchanged
- [ ] `TenantRateLimiter::try_acquire()` returns `TidalError::QuotaExceeded` within 1ms when token bucket is empty
- [ ] Token bucket refills at the configured rate: after sleeping `1/rate` seconds, one token is available
- [ ] WAL directory for `TenantId::DEFAULT` is `{data_dir}/wal/` (unchanged from m1p5)
- [ ] WAL directory for `TenantId(42)` is `{data_dir}/tenants/42/wal/`
- [ ] Unit test: configure 100 signals/sec, write 200 signals in a tight loop, verify ~100 succeed and ~100 receive `QuotaExceeded`
- [ ] `cargo clippy -D warnings` and `cargo fmt` pass
