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>

lib/js/test262-runner.py

@@ -798,6 +798,7 @@ class ServerSession:
         self._run_and_collect(3, '(load "lib/js/parser.sx")', timeout=60.0)
         self._run_and_collect(4, '(load "lib/js/transpile.sx")', timeout=60.0)
         self._run_and_collect(5, '(load "lib/js/runtime.sx")', timeout=60.0)
+        self._run_and_collect(50, '(load "lib/js/regex.sx")', timeout=60.0)
         # Preload the stub harness — use precomputed SX cache when available
         # (huge win: ~15s js-eval HARNESS_STUB → ~0s load precomputed .sx).
         cache_rel = _harness_cache_rel_path()