Add FAISS-backed ACMS read and write adapters on top of the shared VectorStoreService, wire them through the DI container, and cover indexing, scoped search, removal, and benchmark behavior with Behave and ASV.
Address review feedback by keeping the commit scoped to the FAISS backend work and replacing the loose vector-store cache typing with explicit FAISS store protocols instead of Any.
ISSUES CLOSED: #871
