Restore Postgres Bm25 Topk Search

Why

The reference Postgres lexical search path is now fast and honest, but broad queries can still rank only a bounded candidate window. That is an interim quality compromise: callers can tell recall is bounded, but the implementation can still miss a better lexical match outside the window.

What Changes

• Add an optional Postgres pg_search / ParadeDB BM25 lexical backend path for exact scoped top-k retrieval when the extension is explicitly enabled and available.
• Preserve the current native Postgres FTS path as the default fallback with its existing candidate_window recall disclosure.
• Keep SQLite behavior unchanged while preserving API contract parity across SQLite, native Postgres FTS, and optional Postgres BM25.
• Add diagnostics/capability metadata that distinguishes disabled, unavailable, enabled, and fallback_native_fts Postgres lexical backend states.
• Require an explicit deployment/licensing posture before the reference Docker image depends on or bundles pg_search.

Capabilities

New Capabilities

None.

Modified Capabilities

• reference-implementation-architecture: define the optional Postgres BM25 lexical backend, fallback behavior, diagnostics, and deployment constraints.
• lexical-retrieval: define when lexical recall metadata may report exact scoped top-k retrieval instead of a bounded candidate window.

Impact

• Runtime: reference-implementation/server/postgres-search.js, reference-implementation/server/postgres-storage.js, reference-implementation/server/search.js, metadata/diagnostics surfaces.
• Storage: optional Postgres extension probe and optional BM25 index DDL; no SQLite storage change.
• Contracts: lexical recall metadata remains mandatory; optional BM25 may change meta.recall.ranking_scope only when exact scoped top-k is proven.
• Deployment: reference image/operator documentation must explicitly handle pg_search extension availability and licensing posture.
• Tests: Postgres fallback tests, optional pg_search integration tests, lexical retrieval conformance, MCP recall mirroring, and SQLite parity.