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/packages/game_package_manifest.dart

@@ -15,6 +15,7 @@ class GamePackageManifest {
     this.display = const GameDisplayConfig(),
     this.resources = const {},
     this.modules = const {},
+    this.base,
   });
 
   final String gameId;
@@ -29,6 +30,9 @@ class GamePackageManifest {
   final Map<String, GameResource> resources;
   final Map<String, String> modules;
 
+  /// 依赖的框架包 gameId。加载时会先加载框架包，再加载游戏包。
+  final String? base;
+
   static GamePackageManifest fromJsonString(String source) {
     return fromMap(jsonDecode(source) as Map<String, Object?>);
   }
@@ -58,6 +62,8 @@ class GamePackageManifest {
       }
     }
 
+    final base = map['base'] as String?;
+
     final defaultLocale = (map['defaultLocale'] as String?) ?? 'en';
     final supportedLocales = _stringList(
       map,
@@ -89,6 +95,7 @@ class GamePackageManifest {
       display: display,
       resources: resources,
       modules: modules,
+      base: base,
     );
   }