Fix letrec thunk resolution + compiler letrec support + closure JIT check

Root cause: sf-letrec returns a thunk (for TCO) but the CEK dispatch
wrapped it as a value without evaluating. The thunk leaked as the
return value of letrec expressions, breaking sx-parse and any function
using letrec.

Fix: step-sf-letrec unwraps the thunk into a CEK state, so the last
letrec body expression is properly evaluated by the CEK machine.

Also:
- compile-letrec: two-phase (nil-init then assign) for mutual recursion
- Skip JIT for inner functions (closure.bindings != globals) in both
  vm_call and JIT hook
- vm-reset-fn for sx-parse removed (no longer needed)
- Parser regression test: letrec with mutable pos + recursive sublists

Test results: JS 943/17, OCaml 955/0, Python 747/0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/tests/test-parser.sx

@@ -535,3 +535,36 @@
   (deftest "parse nil is not a symbol"
     (let ((result (first (sx-parse "nil"))))
       (assert-nil result))))
+
+
+;; --------------------------------------------------------------------------
+;; JIT regression: mutable pos shared via upvalues across recursive calls
+;; --------------------------------------------------------------------------
+
+(defsuite "parser-jit-regression"
+  (deftest "letrec parser with mutable pos — recursive sublists"
+    ;; Minimal reproducer for the sx-parse JIT bug.
+    ;; Uses define inside fn (like sx-parse's read-list-loop pattern).
+    (let ((parse-fn (fn (src)
+            (let ((pos 0))
+              (letrec
+                ((read-list (fn ()
+                   (let ((result (list))
+                         (done false))
+                     (define go (fn ()
+                       (when (and (not done) (< pos (len src)))
+                         (let ((ch (nth src pos)))
+                           (set! pos (inc pos))
+                           (cond
+                             (= ch ")") (set! done true)
+                             (= ch "(") (do (append! result (read-list)) (go))
+                             :else (do (append! result ch) (go)))))))
+                     (go)
+                     result))))
+                (set! pos 1)
+                (read-list))))))
+      (let ((r (parse-fn "(a(b)(c))")))
+        (assert (list? r) (str "result should be list, got type=" (type-of r)))
+        (assert-equal 3 (len r))
+        (assert-equal (list "a" (list "b") (list "c")) r))))
+)