Tasks: Leave & Stop-Forward (m9-leave-revocation)

T1: Add Tag::CommunityLeave = 0x11 to storage/keys.rs
T2: Add MembershipStatus, CommunityMembership, serialize/deserialize to entities/community.rs
T3: Add community_leave_status DashMap field to TidalDb (db/mod.rs)
T4: Implement leave_community_layer, rejoin_community_layer, community_layer_status, persist_leave_record (db/community.rs)
T5: Add leave gate to try_cohort_attribution and try_community_forwarding (db/signals.rs)
T6: Add rebuild_community_leave_status to db/state_rebuild.rs and call from from_parts
T7: Write integration tests in tidal/tests/m9_leave_revocation.rs (8 tests)
T8: cargo fmt + cargo clippy -D warnings clean