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>

hosts/ocaml/bin/sx_server.ml

@@ -913,9 +913,8 @@ let register_jit_hook env =
        | Some _ -> None  (* compile failed — CEK handles *)
        | None ->
          if !_jit_compiling then None
-         else if Hashtbl.length l.l_closure.bindings > 0
-                 || l.l_closure.parent <> None then
-           (* Skip JIT for lambdas with closure bindings *)
+         else if l.l_closure.bindings != env.bindings then
+           (* Skip JIT for inner functions — closure != globals *)
            None
          else begin
            let fn_name = match l.l_name with Some n -> n | None -> "?" in

hosts/ocaml/lib/sx_vm.ml

@@ -328,11 +328,10 @@ and vm_call vm f args =
        push vm (Sx_ref.cek_call f (List args))
      | None ->
        if l.l_name <> None
-          (* Skip JIT for lambdas with closure bindings — the closure
-             merging into vm_env_ref produces incorrect variable resolution
-             for inner functions (e.g. parser's read-list-loop). *)
-          && Hashtbl.length l.l_closure.bindings = 0
-          && l.l_closure.parent = None
+          (* Skip JIT for inner functions (closure != globals).
+             The closure merging produces incorrect variable resolution
+             for functions that capture letrec/let-local bindings. *)
+          && l.l_closure.bindings == vm.globals
        then begin
          (* Pre-mark before compile attempt to prevent re-entrancy *)
          l.l_compiled <- Some jit_failed_sentinel;