Implements VectorStore interface contract from master plan 21 §3.4:
- sqlite-vec v0.1.9 extension loaded via createRequire (ESM compat)
- vec0 virtual table with FLOAT[N] dimensions driven by EmbeddingResolution
- Upsert via DELETE+INSERT (vec0 does not support INSERT OR REPLACE)
- BigInt rowids required by vec0 v0.1.9 for primary key insertion
- Hybrid RRF (k=60) fusing FTS5 + vector KNN via UNION ALL + GROUP BY
- FTS join on m.memory_id = fts.rowid (migration 023 bridge column)
- VECTOR_STORE_DISABLE_VEC=true test seam for null-extension path
- sanitizeErrorMessage in 3 error paths (Hard Rule #12)
- Raw SQL exception documented in header comment (Hard Rule #5 §D5)
- 27 unit tests across 5 files; all lint/typecheck/cycles checks pass
