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>

shared/sx/ocaml_bridge.py

@@ -265,12 +265,11 @@ class OcamlBridge:
             _logger.info("Loaded %d definitions from .sx files into OCaml kernel (%d skipped)",
                          count, skipped)
 
-            # Auto-compile all lambdas to bytecode VM
-            try:
-                await self._send('(vm-compile)')
-                await self._read_until_ok(ctx=None)
-            except OcamlBridgeError as e:
-                _logger.warning("VM compilation skipped: %s", e)
+            # Auto-compile disabled for now — compiler needs more features
+            # (closures, scope ops, etc.) before it can safely replace
+            # live functions. Infrastructure is ready:
+            #   await self._send('(vm-compile)')
+            #   await self._read_until_ok(ctx=None)
         except Exception as e:
             _logger.error("Failed to load .sx files into OCaml kernel: %s", e)
             self._components_loaded = False  # retry next time