VM auto-compile infrastructure + disable until compiler is complete

Added vm-compile command: iterates env, compiles lambdas to bytecode,
replaces with NativeFn VM wrappers (with CEK fallback on error).
Tested: 3/109 compile, reduces CEK steps 23%.

Disabled auto-compile in production — the compiler doesn't handle
closures with upvalues yet, and compiled functions that reference
dynamic env vars crash. Infrastructure stays for when compiler
handles all SX features.

Also: added set-nth! and mutable-list primitives (needed by
compiler.sx for bytecode patching). Fixed compiler.sx to use
mutable lists on OCaml (ListRef for append!/set-nth! mutation).

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

hosts/ocaml/lib/sx_primitives.ml

@@ -529,6 +529,17 @@ let () =
     match args with [Dict d] -> List (dict_keys d) | _ -> raise (Eval_error "keys: 1 dict"));
   register "vals" (fun args ->
     match args with [Dict d] -> List (dict_vals d) | _ -> raise (Eval_error "vals: 1 dict"));
+  register "mutable-list" (fun _args -> ListRef (ref []));
+  register "set-nth!" (fun args ->
+    match args with
+    | [ListRef r; Number n; v] ->
+      let i = int_of_float n in
+      let l = !r in
+      r := List.mapi (fun j x -> if j = i then v else x) l;
+      Nil
+    | [List _; _; _] ->
+      raise (Eval_error "set-nth!: list is immutable, use ListRef")
+    | _ -> raise (Eval_error "set-nth!: expected (list idx val)"));
   register "dict-set!" (fun args ->
     match args with
     | [Dict d; String k; v] -> dict_set d k v; v