Set arity in native compiler and bytecode modules

Sx_compiler.compile/compile_module now emit arity (local slot count) in
the bytecode dict. MCP sx_build_bytecode serializes arity into .sxbc.json.
sx-platform.js passes arity through to K.loadModule(). Without this, the
VM allocated only 16 local slots per module frame.

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

shared/static/wasm/sx-platform.js

@@ -245,6 +245,7 @@
 
       var module = {
         _type: 'dict',
+        arity: json.module.arity || 0,
         bytecode: { _type: 'list', items: json.module.bytecode },
         constants: { _type: 'list', items: json.module.constants.map(deserializeConstant) },
       };