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/evaluator.sx

@@ -896,6 +896,14 @@
 ;;   (call-thunk f env)         — call a zero-arg function
 ;; --------------------------------------------------------------------------
 
+;; step-sf-letrec: sf-letrec evaluates bindings + intermediate body,
+;; returns a thunk for the last body expression. Unwrap into CEK state
+;; so the last expression is properly evaluated by the CEK machine.
+(define step-sf-letrec
+  (fn (args env kont)
+    (let ((thk (sf-letrec args env)))
+      (make-cek-state (thunk-expr thk) (thunk-env thk) kont))))
+
 (define sf-dynamic-wind
   (fn ((args :as list) (env :as dict))
     (let ((before (trampoline (eval-expr (first args) env)))
@@ -1215,7 +1223,7 @@
               (= name "quasiquote") (make-cek-value (qq-expand (first args) env) env kont)
               (= name "->")     (step-sf-thread-first args env kont)
               (= name "set!")   (step-sf-set! args env kont)
-              (= name "letrec") (make-cek-value (sf-letrec args env) env kont)
+              (= name "letrec") (step-sf-letrec args env kont)
 
               ;; Continuations — native in CEK
               (= name "reset")  (step-sf-reset args env kont)