feat(lab): anchor SECT-INDEX entries by bare numeral + heading confirmation

Documents that number sections bare (**1.1** "Affiliate" means...)
never carry the synthesized kind word, so anchorDisplayLineStart's
startsWith(display) could never match them - whole contract corpora
measured 0% addressable (tax-TP 0/25 docs) and the no-fit band was
closed to the index arm. Adds a digit-led numeral variant with a token
boundary (1.1 never claims 1.10), accepted only when the line continues
heading-like or carries the section's known title (normalized compare,
bold/smart-quote tolerant), plus the pandoc '---' em-dash rendering as a
heading continuation.

No-fit corpus addressability before -> after: CoC 67%->80%, antitrust
10%->100% (of extractable docs), diligence 57%->93%, indenture 68%->72%,
tax-TP 0%->87%. Anchor oracle over all 5 corpora: 5,482 anchors, 0
regressions (3 flagged ARTICLE anchors reproduce on the pre-fix module;
2 new flags are correct-row table-derived subsections reached only
because their parents now anchor). Fit-band invariants probe green
(prefix/kind-filter/idempotency), structureIndexExperiment +
localToolWiring suites 39/39, tsc clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6