feat: multi-package loading with base framework support

- Add RuntimeOptions.basePackages for loading framework packages before game package
- Add ScriptEngine.loadPackages() for multi-package module merging
- LuaDardoScriptEngine merges modules from all packages, game overrides framework
- PackageActivationController loads base packages first, then game package
- GamePackageManifest parses optional 'base' field
- Update docs: README, quick-start, lua-package-format, architecture
- Update all test mocks with loadPackages() implementation

test/runtime/events/runtime_event_dispatcher_test.dart

@@ -156,6 +156,12 @@ class _FakeScriptEngine implements ScriptEngine {
     RuntimeScriptServices services = const RuntimeScriptServices(),
   }) async {}
 
+  @override
+  Future<void> loadPackages(
+    List<GamePackage> packages, {
+    RuntimeScriptServices services = const RuntimeScriptServices(),
+  }) async {}
+
   @override
   bool smokeTest(Map<String, Object?> context) => true;