feat(m9): implement purge re-materialization engine
Adds the M9 purge re-materialization feature: a WAL-replay background
engine that rebuilds community cohort aggregates for a (user, community)
pair after retroactive signal purge, restoring ranking correctness without
modifying the immutable WAL.
Key additions:
- cohort::rematerialization module: PurgeJobQueue, RematerializationEngine,
WAL replay, atomic CohortSignalLedger swap, BLAKE3 audit log, metrics counters
- TidalDb::{submit_purge_job, purge_job_status, rematerialization_metrics}
public API (db/rematerialization.rs)
- Engine auto-starts in persistent mode; clean shutdown before WAL teardown
- 6 integration tests in tests/m9_purge_remat.rs covering ephemeral and
persistent modes, job lifecycle, and multi-job independence
- Split oversized files to comply with 600-line limit: db/mod.rs →
db/from_parts.rs, entities/revocation.rs → revocation/{mod,tests}.rs,
schema/validation/builders.rs → builders/{mod,tests}.rs,
signals/warm.rs → warm/{mod,tests,proptests}.rs
- Fix pre-existing bootstrap errors: export AuditKind from session module,
add overrides_rejected to SessionSnapshot deserialization
6f26d03c77