[YBazanPro]

Stacked on #20 (spec 020): this PR's base is feat/spec-020-first-release-polish, because the change touches files spec 020 also changed.

Problem

The hybrid keyword pool contributed nothing:

• AND semantics: websearch_to_tsquery ANDs every word, so "How long do I have to submit an expense claim after a trip?" only matched a passage holding all of claim, expens, long, submit, trip. On the sample corpus, typical golden questions got 0 keyword hits.
• Cosine floor: with reranking off or skipped, keyword-only hits still had to pass the cosine floor. It dropped exactly the exact-term matches hybrid search exists for.

A baseline run --variants on the current code confirms it: vector and hybrid are identical (R@5 0.86, MRR 0.84, behaviour 0.83, refusals 0.80).

Change

• OR query: the question's distinct lexemes are OR-ed into one tsquery, still ranked with ts_rank_cd. They are quoted in as Postgres produced them, so nothing stems them a second time.
• Coverage: each keyword match records the share of the question's lexemes the passage contains.
• Floor exemption: while cosine is the gate, a match with coverage ≥ Documents:Retrieval:Hybrid:MinKeywordCoverage (default 0.5) passes the cosine floor. One common word is not evidence: on the dev corpus, "Can I bring my dog to the office?" matches 6 passages through offic alone, at coverage 0.33. ConfidentScore still applies, so keyword-only evidence is LowConfidence. When reranking is applied, the rerank score gates as before.
• Visibility: KeywordCoverage is a new stage field on GET /api/documents/search. The setting shows in the admin configuration and in harness reports.
• Docs: spec 019 (scope, evidence gate, edge cases, AC-23 to AC-25) and CLAUDE.md are updated.

Verification

• dotnet build -c Release: 0 warnings. dotnet format --verify-no-changes: clean.
• Documents unit tests: 305/305.
• Integration: HybridSearchTests 14/14, plus the other retrieval classes (reranking, contract, endpoint, visibility, candidates, retrieval stages, suggested questions, TEI) 76/76. The full integration suite was not run locally; CI runs it.

Not done yet

• The variants have not been rerun on this code, so there is no "after" number. The baseline run stopped before the hybrid + rerank variant. The calibration section of spec 019 (AC-20 to AC-22) is still open, and MinKeywordCoverage = 0.5 is provisional until that run.
• KeywordCoverage is not yet in the chat request trace or the admin inspector, only on the diagnostic endpoint.

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