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/runtime.sx

@@ -2032,7 +2032,15 @@
           (&rest args)
           (cond
             ((= (len args) 0) nil)
-            ((js-regex? (nth args 0)) (js-regex-stub-exec (nth args 0) s))
+            ((js-regex? (nth args 0))
+              (let
+                ((rx (nth args 0)))
+                (let
+                  ((impl (get __js_regex_platform__ "exec")))
+                  (if
+                    (js-undefined? impl)
+                    (js-regex-stub-exec rx s)
+                    (impl rx s)))))
             (else
               (let
                 ((needle (js-to-string (nth args 0))))
@@ -2041,7 +2049,7 @@
                   (if
                     (= idx -1)
                     nil
-                    (let ((res (list))) (append! res needle) res))))))))
+                    (let ((res (list))) (begin (append! res needle) res)))))))))
       ((= name "at")
         (fn
           (i)
@@ -2099,6 +2107,20 @@
       ((= name "toWellFormed") (fn () s))
       (else js-undefined))))
 
+(define __js_tdz_sentinel__ (dict "__tdz__" true))
+
+(define js-tdz? (fn (v) (and (dict? v) (dict-has? v "__tdz__"))))
+
+(define
+  js-tdz-check
+  (fn
+    (name val)
+    (if
+      (js-tdz? val)
+      (raise
+        (TypeError (str "Cannot access '" name "' before initialization")))
+      val)))
+
 (define
   js-string-slice
   (fn