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/conformance.sh

@@ -49,6 +49,8 @@ trap "rm -f $TMPFILE" EXIT
   echo '(load "lib/js/transpile.sx")'
   echo '(epoch 5)'
   echo '(load "lib/js/runtime.sx")'
+  echo '(epoch 6)'
+  echo '(load "lib/js/regex.sx")'
 
   epoch=100
   for f in "${FIXTURES[@]}"; do