D2 close — fossic-similarity-hnsw substrate complete
Sibling crate ships with HNSW-backed semantic search via SimilaritySearchProvider trait, atomic disk persistence, background indexing, Python bindings, and full documentation.
·Substrate extension pattern validated: lazy Mutex<Option<SystemStreamWriter>> from Phase 6/7/8 transferred cleanly to D2 sibling crate, now an established substrate idiom
·Five SystemStreamWriter instances in steady state: dispatcher, reducer registry, background executor, project registry, HNSW provider — all using the same lazy-init pattern
·Cerebra and other consumers can register HnswProvider via OpenOptions::similarity_provider to enable substrate-level semantic search without the 50k-embedding ceiling of in-memory implementations