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

lib/runtime/game/runtime_options.dart

@@ -1,7 +1,14 @@
 class RuntimeOptions {
-  const RuntimeOptions({this.runtimeLuaRoot = defaultRuntimeLuaRoot});
+  const RuntimeOptions({
+    this.runtimeLuaRoot = defaultRuntimeLuaRoot,
+    this.basePackages = const [],
+  });
 
   static const defaultRuntimeLuaRoot = 'assets/runtime/lua';
 
   final String runtimeLuaRoot;
+
+  // 框架包 gameId 列表，按顺序先于游戏包加载。
+  // 后加载的同名模块覆盖先加载的。
+  final List<String> basePackages;
 }