Spec deltas

Lexical retrieval responses SHALL report exact recall only when the active backend ranks the full grant-authorized lexical match set before pagination. A backend that applies a bounded candidate window, approximate prefilter, or any unproven pre-ranking truncation SHALL report incomplete recall using meta.recall.rankingscope: "candidatewindow" or "unknown" as appropriate.

The reference implementation SHALL treat any pg_search / ParadeDB BM25 lexical backend as an optional Postgres runtime capability. The default Postgres lexical backend SHALL remain the native scoped FTS path unless configuration explicitly enables the BM25 backend and startup proves the required extension and index are usable. If the BM25 backend is disabled, unavailable, not ready, or fails at query time, the reference SHALL fall back to native Postgres lexical retrieval without changing the public /v1/search response shape.