fix(scan): prune a tier-zero question unless ITS partition is the incomplete one

The mechanism already existed. A question whose evidence vector says no sink of its family is in
reach cannot produce a flow, and tier-zero pruning defers it. That was switched off for the whole
scan whenever any of twelve degradation reasons appeared, and a multi-language repository always
carries two of them, so the measured plugin arbitrated every question it could ask. Third instance
this week of one blast radius, and the invariant written into the design on Monday named it.

A prune blocker that names a partition now withholds pruning from that partition questions and from
nobody else. cross_partition_semantics_unresolved leaves the blocker set entirely, for the reason it
left the integrity gaps: a PHP family sink is PHP, so a second language cannot be why its vector saw
none.

Measured on Paid Memberships Pro. At 25 regions arbitration falls from 127 questions to 34 with 93
recorded deferred as tier_zero, and 19 findings before and after, identical family by family. At 500
it falls from 2,526 to 406 with 2,120 deferred.

It is not sufficient, and that is the useful half. The 500-region run still fails in the same way at
the same cost, because 406 questions at roughly one and a half seconds each still exceed the
300-second per-batch ceiling. This lever moved the volume; the ceiling is a separate limit.

The census now records what a scan skipped and why, per family, so a cheaper scan and a narrower one
stop looking identical: a family whose every question was pruned reports no sink of this family in
reach rather than region offered, no question asked.

1,420 tests pass, eighteen skipped.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.kiro/specs/contributor-scan/tasks.md

@@ -1239,6 +1239,9 @@ the ontology describes libraries, frameworks and language constructs, never repo
   - The structural pass answers 17,700 requests in 44 s; arbitration costs one to two seconds per question, and a 673-file plugin asks 2,526 of them. Gate arbitration on the evidence vector already computed: a question whose scope holds no modelled sink, or no reachable source, cannot produce a flow and does not need the expensive pass.
   - This attacks the scale limit from the opposite side to graph reuse and can be sized today from data already on disk, by counting how many of those 2,526 questions had any sink in scope at all.
   - Done when the arbitrated count falls without any finding being lost on the measured subjects, and the questions skipped are recorded as skipped rather than as answered.
+  - _Implemented and measured 2026-09-22, and the mechanism already existed._ Tier-zero pruning defers a question whose evidence vector says no sink of its family is in reach. It was switched off for the WHOLE SCAN whenever any of twelve degradation reasons appeared, and a multi-language repository always carries two of them, so the measured plugin arbitrated every question it could ask. Third instance this week of the same blast radius. A prune blocker that names a partition now withholds pruning from that partition's questions and nobody else's, and `cross_partition_semantics_unresolved` leaves the blocker set entirely for the reason it left the integrity gaps.
+  - _The arbitrated count falls and no finding is lost._ At 25 regions: 127 questions to 34, with 93 recorded deferred as `tier_zero`, and 19 findings before and after, identical family by family. At 500: 2,526 to 406, with 2,120 deferred. A family whose every question was pruned now reports "no sink of this family in reach" rather than "region offered, no question asked", because nothing to ask is a different statement from never offered.
+  - **It is not sufficient, and that is the useful half of the result.** The 500-region run still fails in the same way at the same cost, because 406 questions at roughly one and a half seconds each still exceed the 300-second per-batch ceiling. This lever moved the volume; the ceiling is a separate limit and pre-push 13.3 still cannot reach the population that holds its target without it. Evidence: `benchmarks/measurements/2026-09-22-tier-zero-pruning.json`.
   - _Boundary: contributor-scan scan budget_
   - _Requirements: 4.1, 6.3, 8.1_
 - [ ] 7.4 One matcher, used by every query