js: regex engine (lib/js/regex.sx) — pure-SX recursive backtracker
Adds a full regex engine written in SX, installed via js-regex-platform-override!.
Supports char classes (. \d\D\w\W\s\S [abc] [^abc] ranges), anchors (^ $ \b \B),
quantifiers (* + ? {n,m} greedy and lazy), capturing/non-capturing groups,
alternation (a|b), flags i/g/m. exec() returns {:match :index :input :groups}.
Also fixes String.prototype.match to dispatch through the platform engine
(was calling js-regex-stub-exec directly, bypassing regex.sx).
Adds TDZ sentinel infrastructure: __js_tdz_sentinel__, js-tdz?, js-tdz-check.
Updates test.sh (+34 regex tests + 4 TDZ infra tests), conformance.sh,
and test262-runner.py to load regex.sx as epoch 6.
Tests: 559/560 unit (1 pre-existing failure), 148/148 conformance.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
giles
f8023cf74e