Epic: retrieval-diagnostics — explain-miss and the floor-ratio boost gate

[tcballard]

Execution tracking (epic) for the retrieval-diagnostics roadmap (ADR-093: intent lives in the corpus, execution here).

Intent (corpus): rac/roadmaps/retrieval-diagnostics.md — the rank-8 Tranche B item of the deterministic-substrate programme (#248), graduated from future/ now its sequencing condition is met: relevance ranking (ADR-078) is Achieved, so the gate has a shipped boost to bound and explain-miss has a shipped explain-hit to mirror. Both initiatives stay inside the no-embeddings, no-LLM line (ADR-066, ADR-002): pure deterministic traces over the existing matcher and scorer.

Taken into build for the July release train (scheduled across 2026.07.3–2026.07.4).

Initiatives (each a sub-issue, paired requirement in the corpus):

• retrieval-diagnostics: explain-miss — deterministic "why not X" trace #315 — explain-miss (diagnose): deterministic "why did my query not surface artifact X" trace, complementing explain-hit (rac-explain-miss-diagnostics)
• retrieval-diagnostics: floor-ratio boost gate — the bounded-boost invariant as a tested gate #316 — floor-ratio boost gate: the ADR-078 bounded-boost invariant as a tested, golden-pinned gate, not a coefficient property (rac-floor-ratio-boost-gate)

Initiative 3 (the two-tier recall framing) is a documentation deliverable recorded in the roadmap, not tracked as a build sub-issue.

Ordering and task state live here; the roadmap remains the durable record of what and why.