Generalizes the crown repro (Worldtree #400 / thread 01KZETD98T) beyond its anchor
into a before/after regression instrument for the class property: when the corpus
holds a chunk whose text literally carries a queried surface form, a natural query
should serve >=1 such chunk at a usable rank.
- Anchors tagged stress (common word + competing dense cluster, e.g. crown) vs
control (distinctive name — should sit ~0% miss).
- Binary per trial: does a natural query serve >=1 term-containing chunk within
top-10 (USABLE_K)? Ranks >=8 flagged KNIFE-EDGE (the RRF fused-rank 9-11 window
residual worldtree-dev's decomposition measured).
- Real-world end-to-end: drives the agent (it composes its own reference_knowledge
query, as in production); --runs samples query-formulation variance to estimate a
true miss-rate.
- Extensible anchor list; --anchor filters.
This is the deciding instrument for the rerank_hybrid_floor lever: its stress-class
miss-rate (alongside brokkr's fleet demotion rate) rules the floor in or out after
the BM25 stemming fold deploys. Pre-fold baseline captured today (the "before"):
control 0% miss / stress[crown] 100% miss / 0% knife-edge, 11 trials.
Diagnostics fixture, no production runtime — no version bump. persistent-memory
snapshot committed alongside (commit-along).
