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/bin/run_tests.ml

@@ -637,12 +637,24 @@ let run_spec_tests env test_files =
   Printf.printf "\nLoading test framework...\n%!";
   load_and_eval framework_path;
 
+  (* Load compiler + VM spec for test-vm.sx *)
+  let spec_dir = Filename.concat project_dir "spec" in
+  List.iter (fun name ->
+    let path = Filename.concat spec_dir name in
+    if Sys.file_exists path then begin
+      Printf.printf "Loading %s...\n%!" name;
+      (try load_and_eval path
+       with e -> Printf.eprintf "Warning: %s: %s\n%!" name (Printexc.to_string e))
+    end
+  ) ["bytecode.sx"; "compiler.sx"; "vm.sx"];
+
   (* Determine test files *)
   let files = if test_files = [] then begin
     let entries = Sys.readdir spec_tests_dir in
     Array.sort String.compare entries;
     let requires_full = ["test-continuations.sx"; "test-types.sx"; "test-freeze.sx";
-                          "test-continuations-advanced.sx"; "test-signals-advanced.sx"] in
+                          "test-continuations-advanced.sx"; "test-signals-advanced.sx";
+                          "test-vm.sx"] in
     Array.to_list entries
     |> List.filter (fun f ->
       String.length f > 5 &&