retrieval-diagnostics: floor-ratio boost gate — the bounded-boost invariant as a tested gate

[tcballard]

Sub-issue of #314 (retrieval-diagnostics, ADR-093).

Corpus: rac/requirements/rac-floor-ratio-boost-gate.md (Proposed) — Initiative 2 of the roadmap.

Scope:

• ADR-078's deterministic relevance score includes a bounded graph boost; today "bounded" is a property of the coefficients, not a stated invariant. Make it a gate: no boosted result may outrank a stronger lexical match beyond the bounded ratio.
• Proven by golden tests, so a future retuning of the boost fails loudly instead of silently degrading grounding — the exact failure hybrid systems guard against with a floor-ratio gate.
• Ranking goldens stay byte-stable for corpora where the gate never triggers; deterministic and offline throughout (ADR-066).

Tick on #314 when the gate is golden-pinned in CI.