vm.sx feature parity: JIT dispatch, active VM tracking, CEK fallback

Add missing features to lib/vm.sx that sx_vm.ml has:
- *active-vm* mutable global for HO primitive callback VM reuse
- *jit-compile-fn* platform-settable JIT compilation hook
- try-jit-call: check lambda-compiled, attempt JIT, fallback to CEK
- vm-call: VmClosure→push-frame, Lambda→try-jit, Component→CEK
- vm-call-closure: save/restore *active-vm* around execution
- vm-push-frame: refactored to use accessor functions
- cek-call-or-suspend: preamble-provided CEK interop

Transpiled output (sx_vm_ref.ml) now has 12 functions (was 9):
  *active-vm*, *jit-compile-fn*, try-jit-call, vm-call,
  vm-resolve-ho-form, vm-call-external, env-walk, env-walk-set!,
  vm-run, vm-step, vm-call-closure, vm-execute-module

48 preamble functions (native OCaml type access).

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

hosts/ocaml/bootstrap_vm.py

@@ -70,6 +70,10 @@ SKIP = {
     # Complex native ops
     "vm-push-frame", "code-from-value", "vm-closure?",
     "vm-create-closure",
+    # Lambda accessors (native type)
+    "lambda?", "lambda-compiled", "lambda-set-compiled!", "lambda-name",
+    # CEK interop
+    "cek-call-or-suspend",
     # Collection helpers (use mutable state + recursion)
     "collect-n-from-stack", "collect-n-pairs", "pad-n-nils",
 }