spec+docs(search-with-proof): record first cluster demonstration + fixed-side negative [skip ci]

Feed the milestone back into the spec and steering. HANDOFF: the open "first demonstrated"
question is RESOLVED -- xiaomusic (path, prefix-check-bypass) demonstrated on the vulnerable
side (3/3 affected, 0/3 safe) and the blind fixed-side search produced 0 false proofs; pair
total $1.22. Records the six-fix chain that enabled it (parity, verify-time-only, fair-share
budget, served-root guidance, oracle prefix-bypass coverage, control-signal feedback) plus the
keystone. tasks.md: implementation note; pilot (6.2/6.3) in progress. spec.json: pilot status.
safety-net steering: proof-based BLOCK mechanism reached first execution-backed demonstration
(feasibility-pilot scope, not release readiness; pilot exit + 73-negative qualification open;
classifier stays triage). search README: status note. Payload-free; demos/boards stay gitignored.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

benchmarks/search/README.md

@@ -131,3 +131,12 @@ manifest; without a subtype verification returns `no_oracle`. No subtype is gues
 from repository names. The selected manifest's pair 1 currently lacks this metadata.
 Live transport, provider authentication/cost, real project buildability, and AX
 oracle operation must still be measured; an offline probe cannot establish them.
+
+## Status (2026-10-07)
+
+The feasibility pilot achieved its first cluster `demonstrated` finding (xiaomusic, path /
+prefix-check-bypass): vulnerable side demonstrated, fixed side blind search produced no
+false proof. See `benchmarks/measurements/2026-10-07-search-first-cluster-demonstration/`.
+Outcomes and spend are aggregates only; stored demos and boards stay in the gitignored
+`benchmarks/search/private/` area. Pilot exit (≥5 demonstrations / 0 false proofs per
+family) and the ≥73-negative BLOCK qualification remain open.