VM spec in SX + 72 tests passing on both JS and OCaml

spec/vm.sx — bytecode VM written in SX (the spec):
  - Stack-based interpreter for bytecode from compiler.sx
  - 24 opcodes: constants, variables (local/upvalue/global), control flow,
    function calls (with TCO), closures with upvalue capture, collections,
    string concat, define
  - Upvalue cells for shared mutable closure variables
  - Call dispatch: vm-closure (fast path), native-fn, CEK fallback
  - Platform interface: 7 primitives (vm-stack-*, call-primitive, cek-call,
    get-primitive, env-parent)

spec/tests/test-vm.sx — 72 tests exercising compile→bytecode→VM pipeline:
  constants, arithmetic, comparison, control flow (if/when/cond/case/and/or),
  let bindings, lambda, closures, upvalue mutation, TCO (10K iterations),
  collections, strings, define, letrec, quasiquote, threading, integration
  (fibonacci, recursive map/filter/reduce, compose)

spec/compiler.sx — fix :else keyword detection in case/cond compilation
  (was comparing Keyword object to evaluated string, now checks type)

Platform primitives added (JS + OCaml):
  make-vm-stack, vm-stack-get, vm-stack-set!, vm-stack-length, vm-stack-copy!,
  primitive?, get-primitive, call-primitive, set-nth! (JS)

Test runners updated to load bytecode.sx + compiler.sx + vm.sx for --full.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -594,7 +594,8 @@ let () =
     | _ -> raise (Eval_error "error: 1 arg"));
   register "apply" (fun args ->
     match args with
-    | [NativeFn (_, f); List a] -> f a
+    | [NativeFn (_, f); (List a | ListRef { contents = a })] -> f a
+    | [NativeFn (_, f); Nil] -> f []
     | _ -> raise (Eval_error "apply: function and list"));
   register "identical?" (fun args ->
     match args with [a; b] -> Bool (a == b) | _ -> raise (Eval_error "identical?: 2 args"));
@@ -668,4 +669,53 @@ let () =
       Bool (List.for_all (fun x -> sx_truthy (call_any f [x])) items)
     | [_; Nil] -> Bool true
     | _ -> raise (Eval_error "every?: expected (fn list)"));
+
+  (* ---- VM stack primitives (vm.sx platform interface) ---- *)
+  register "make-vm-stack" (fun args ->
+    match args with
+    | [Number n] -> ListRef (ref (List.init (int_of_float n) (fun _ -> Nil)))
+    | _ -> raise (Eval_error "make-vm-stack: expected (size)"));
+  register "vm-stack-get" (fun args ->
+    match args with
+    | [ListRef r; Number n] -> List.nth !r (int_of_float n)
+    | _ -> raise (Eval_error "vm-stack-get: expected (stack idx)"));
+  register "vm-stack-set!" (fun args ->
+    match args with
+    | [ListRef r; Number n; v] ->
+      let i = int_of_float n in
+      r := List.mapi (fun j x -> if j = i then v else x) !r; Nil
+    | _ -> raise (Eval_error "vm-stack-set!: expected (stack idx val)"));
+  register "vm-stack-length" (fun args ->
+    match args with
+    | [ListRef r] -> Number (float_of_int (List.length !r))
+    | _ -> raise (Eval_error "vm-stack-length: expected (stack)"));
+  register "vm-stack-copy!" (fun args ->
+    match args with
+    | [ListRef src; ListRef dst; Number n] ->
+      let count = int_of_float n in
+      let src_items = !src in
+      dst := List.mapi (fun i x -> if i < count then List.nth src_items i else x) !dst; Nil
+    | _ -> raise (Eval_error "vm-stack-copy!: expected (src dst count)"));
+  register "primitive?" (fun args ->
+    match args with
+    | [String name] -> Bool (Hashtbl.mem primitives name)
+    | _ -> Bool false);
+  register "get-primitive" (fun args ->
+    match args with
+    | [String name] ->
+      (match Hashtbl.find_opt primitives name with
+       | Some fn -> NativeFn (name, fn)
+       | None -> raise (Eval_error ("VM undefined: " ^ name)))
+    | _ -> raise (Eval_error "get-primitive: expected (name)"));
+  register "call-primitive" (fun args ->
+    match args with
+    | [String name; (List a | ListRef { contents = a })] ->
+      (match Hashtbl.find_opt primitives name with
+       | Some fn -> fn a
+       | None -> raise (Eval_error ("VM undefined: " ^ name)))
+    | [String name; Nil] ->
+      (match Hashtbl.find_opt primitives name with
+       | Some fn -> fn []
+       | None -> raise (Eval_error ("VM undefined: " ^ name)))
+    | _ -> raise (Eval_error "call-primitive: expected (name args-list)"));
   ()