docs: relevance floor + silent-vs-always contract spec (#289)

[robotrocketscience]

Summary

Spec memo for #289. Phase-2a of the #286 redesign — the gating contract change so rebuild_v14() can return empty when no candidate clears the floor. Doc-only — no code change. Posted for ratification.

Five recommendations:

1. Yes — add a silent path. All-floored-out → "". No empty-marker tag.
2. Composite score formula. bm25_normalized * (0.5 + 0.5 * posterior_mean). Posterior-mean weighting prevents stale-but-token-overlapping beliefs (the 1bc8ab45a40351d9 example in Context rebuild surfaces duplicate + off-topic beliefs; ranking has no relevance floor or output-dedup #281) from passing.
3. Three-tier per-lane floors. L0 locked = no floor (operator intent dominates). L2 session-scoped = soft floor T_session = 0.10. L1 / L2.5 = hard floor T_l1 = 0.40.
4. No last_retrieved_at stamp on floored-out hits. Stamp moves to after floor application; floored-out beliefs never reach the stamp loop. Protects the hibernation staleness signal (v2.0 substrate decision: multi-axis vs single-axis uncertainty #196).
5. Ship with placeholder T defaults; calibrate from Rebuild redesign: eval harness — per-rebuild log + fixed-corpus precision #288 logs. Floor value blocks on Rebuild redesign: eval harness — per-rebuild log + fixed-corpus precision #288 eval harness data; floor shape lands in v1.x with operator-tunable [rebuild_floor] config.

Decision asks (five)

Bottom of the memo. Composite formula, per-lane structure, empty-path shape, stamp ownership, calibration plan.

What ships if ratified

One PR (~400 lines):

• rebuild_v14 composite + floor + empty-path return
• hook.py precompact-envelope drop on empty
• hook_search.py stamp move (only survivors get stamped)
• [rebuild_floor] config block + tests

Test plan

• Memo cross-references Context rebuild redesign — scoping (parent) #286, Rebuild redesign: eval harness — per-rebuild log + fixed-corpus precision #288, Context rebuild surfaces duplicate + off-topic beliefs; ranking has no relevance floor or output-dedup #281, last_retrieved_at not populated; retrieval-audit-loop not landing on user store #222/fix: stamp beliefs.last_retrieved_at on hook-driven retrieval (#222) #266
• No code change; nothing to test
• Ratification or override comment from maintainer

Refs #289, #286

[robotrocketscience]

[claim:review:Gylf:2026-04-29T02:28:35Z]

[robotrocketscience]

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