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/scripting/script_engine.dart

@@ -4,11 +4,19 @@ import '../packages/game_package.dart';
 import 'runtime_script_services.dart';
 
 abstract interface class ScriptEngine {
+  // 加载单个包（向后兼容，内部调 loadPackages([package])）。
   Future<void> loadPackage(
     GamePackage package, {
     RuntimeScriptServices services = const RuntimeScriptServices(),
   });
 
+  // 加载多个包，按顺序合并模块，后加载的同名模块覆盖先加载的。
+  // 入口脚本使用最后一个包。
+  Future<void> loadPackages(
+    List<GamePackage> packages, {
+    RuntimeScriptServices services = const RuntimeScriptServices(),
+  });
+
   bool smokeTest(Map<String, Object?> context);
 
   GameDiff init(Map<String, Object?> context);