feat: add foreign class support with allocation, construct opcodes, and CLI callback wiring

Implement the initial infrastructure for foreign classes in the Wren VM, including new opcodes (FOREIGN_CONSTRUCT, FOREIGN_CLASS), a WrenBindForeignClassFn callback type in the public API, and updated CLI vm.c to store and forward foreign binding callbacks via setForeignCallbacks(). The compiler now tracks whether a class is foreign via a new isForeign flag in ClassCompiler, emits CODE_FOREIGN_CONSTRUCT instead of CODE_CONSTRUCT for foreign class constructors, and errors on field definitions inside foreign classes. The debug dump and opcode header gain entries for the new opcodes, and wrenBindSuperclass handles the -1 numFields sentinel for foreign classes to prevent field inheritance from superclasses.

test/api/value.c

@@ -15,7 +15,7 @@ static void getValue(WrenVM* vm)
   wrenReleaseValue(vm, value);
 }
 
-WrenForeignMethodFn valueBindForeign(const char* signature)
+WrenForeignMethodFn valueBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Api.value=(_)") == 0) return setValue;
   if (strcmp(signature, "static Api.value") == 0) return getValue;