fix: admit immutable var integer TA lengths

[luisleo526]

Summary

• admit a uniquely bound top-level var int name = <integer literal> as a static TA constructor length while preserving ordinary persistent-state reads
• fail closed on reassignment, history use, source-order violations, shadow/binding ambiguity, user declarations, receiver use, series promotion, and parser recovery
• narrow the request.security / request.security_lower_tf mutable exemption to admitted TA constructor arguments only
• compile the existing 17-bos-curv Pine v6 benchmark with exact pivot lengths (5, 5) instead of rejecting the strategy

Validation

• focused stable-var tests: 55/55
• exact full native suite: 2,282 passed, 1 skipped
• standalone immutable corpus: 254/254
• Pyodide gate: 277 fixtures, 263/263 supported plus 14 expected errors
• exact 915-source differential: 902 unchanged successes, 12 unchanged errors, and only 17-bos-curv changes from error to success
• independent candidate review: GO
• Git-anchored N=4 preregistration: scraper PR #109, merge 54b58148..., main CI green
• authoritative exhaustive matrix: fingerprint 6eb9c7fd..., 16/16 passed, zero failed/skipped/resumed, all-off gate true, independent adjudicator GO, domain/publication gates true
• frozen all-on production diff: e1edcb49...; frozen test patch: bd6fb651...

Score impact

This is a Pine v6 correctness/coverage fix. It does not claim a tier movement and does not require a new TradingView export; the public board remains 354 Excellent / 47 Strong / 4 Moderate / 2 Weak, with 7 excluded.