sweep hygiene: reservoir-sampled failures file; CHRT/CITT stub sentinels

Two instrument defects from the full-run vet (task #28 item 7):

1. The failures file was a head-cap: the first 2,000 failing docs in
   scan order, so the vet queue only ever showed the alphabetically
   first courts and the corpus tail never appeared. Both pool shapes now
   reservoir-sample (Vitter's R, seed 47) inside _fold_record, and the
   sharded merge allocates the 2,000 slots across shard reservoirs
   proportionally to each shard's failure count (largest remainders fill
   the slack) - a uniform sample over ALL failures; the old merge kept
   whatever the first shards to finish had head-capped.

2. CHRT/CITT provider catalog cards whose Decision Content section
   states "There is no document available for this decision." / "Il n'y
   a pas de document disponible pour cette décision." (en 185 + fr 186
   docs, probed 2026-07-30) were counted as no_addressable_structure
   failures. They now classify as structure kind stub_sentinel - a
   provider inventory fact, not a detector miss - matched only in the
   document tail where the provider layout puts the sentinel, so a
   decision QUOTING the phrase stays a real document.

Verification: smoke-tier cases sweep green (5,096 docs, 77.8s; the
1,331-row failures file now spans 25 distinct courts). Targeted scan of
20 real sentinel docs across CHRT/CITT en+fr: 20/20 classify
stub_sentinel with zero fail flags; a long real CHRT decision and the
same decision with the sentinel phrase spliced MID-text both stay
"paragraphs" (the tail anchor holds). The shard-vs-legacy equivalence
gate is untouched by the sampling change - both pool shapes fold
through the same _fold_record and the compared aggregates carry no
failure rows.

Left open from item 7, deliberately: the competition-rule redesign for
the three true mis-selection classes (length-beats-style hijack,
gap-splice chimera, collapsed-fragment lock). That is a measured
detector change gated on the narrow-flag queue as its eval set - the
same discipline as dbb7b355 - not an instrument fix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012AgUm5EcRKbT3duVFMbrQD