VM: closure env chain for GLOBAL_GET/SET + remove JIT skip

vm_closure now stores the original closure env (vm_closure_env).
GLOBAL_GET walks the closure env chain when the variable isn't in
vm.globals. GLOBAL_SET writes to the correct env in the chain.

This enables JIT compilation of all named functions regardless of
closure depth. No more closure skip check needed.

Pre-compile time back to ~7s (was 37s with closure skip).

Note: sx-parse sibling list parsing still has issues — the root
cause is in how the JIT-compiled letrec + OP_CLOSURE interacts
with the upvalue cell mechanism. Investigation ongoing.

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

hosts/ocaml/bin/sx_server.ml

@@ -913,9 +913,6 @@ let register_jit_hook env =
        | Some _ -> None  (* compile failed — CEK handles *)
        | None ->
          if !_jit_compiling then None
-         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
            _jit_compiling := true;
@@ -1300,7 +1297,7 @@ let rec dispatch env cmd =
                   try
                     Sx_vm.call_closure
                       { vm_code = code; vm_upvalues = [||]; vm_name = lam.l_name;
-                        vm_env_ref = live_env }
+                        vm_env_ref = live_env; vm_closure_env = None }
                       args live_env
                   with
                   | _ ->