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/lua_dardo_script_engine.dart

@@ -27,15 +27,34 @@ class LuaDardoScriptEngine implements ScriptEngine {
   Future<void> loadPackage(
     GamePackage package, {
     RuntimeScriptServices services = const RuntimeScriptServices(),
+  }) {
+    return loadPackages([package], services: services);
+  }
+
+  @override
+  Future<void> loadPackages(
+    List<GamePackage> packages, {
+    RuntimeScriptServices services = const RuntimeScriptServices(),
   }) async {
+    if (packages.isEmpty) {
+      throw const FormatException('loadPackages requires at least one package');
+    }
+
     _services = services;
     _networkRequestCounter = 0;
     _hostCallCounter = 0;
-    final script = await package.readText(package.manifest.entry);
     _moduleScripts = {};
-    for (final entry in package.manifest.modules.entries) {
-      _moduleScripts[entry.key] = await package.readText(entry.value);
+
+    // 按顺序加载所有包的模块，后加载的同名模块覆盖先加载的。
+    for (final package in packages) {
+      for (final entry in package.manifest.modules.entries) {
+        _moduleScripts[entry.key] = await package.readText(entry.value);
+      }
     }
+
+    // 入口脚本使用最后一个包（游戏包）。
+    final entryPackage = packages.last;
+    final script = await entryPackage.readText(entryPackage.manifest.entry);
     _loadingModules.clear();
 
     _lua = LuaState.newState();