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>

spec/compiler.sx

@@ -19,7 +19,7 @@
 
 (define make-pool
   (fn ()
-    {:entries (list)
+    {:entries (if (primitive? "mutable-list") (mutable-list) (list))
      :index   {:_count 0}}))
 
 (define pool-add
@@ -92,7 +92,7 @@
 
 (define make-emitter
   (fn ()
-    {:bytecode (list)   ;; list of bytes
+    {:bytecode (if (primitive? "mutable-list") (mutable-list) (list))
      :pool     (make-pool)}))
 
 (define emit-byte