feat: add runtime i18n API with manifest translations

lib/runtime/packages/game_package_manifest.dart

@@ -15,6 +15,7 @@ class GamePackageManifest {
     this.display = const GameDisplayConfig(),
     this.resources = const {},
     this.modules = const {},
+    this.translations = const {},
     this.base,
   });
 
@@ -30,6 +31,10 @@ class GamePackageManifest {
   final Map<String, GameResource> resources;
   final Map<String, String> modules;
 
+  /// 翻译字典：locale → { key → translatedText }。
+  /// Runtime 会按包加载顺序合并，游戏包覆盖框架包。
+  final Map<String, Map<String, String>> translations;
+
   /// 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;
 
@@ -64,6 +69,26 @@ class GamePackageManifest {
 
     final base = map['base'] as String?;
 
+    final translationsValue = map['translations'];
+    final translations = <String, Map<String, String>>{};
+    if (translationsValue is Map) {
+      for (final entry in translationsValue.entries) {
+        if (entry.key is! String || entry.value is! Map) {
+          throw const FormatException('manifest.translations must be a map');
+        }
+        final localeDict = <String, String>{};
+        for (final kv in (entry.value as Map).entries) {
+          if (kv.key is! String || kv.value is! String) {
+            throw const FormatException(
+              'manifest.translations values must be string maps',
+            );
+          }
+          localeDict[kv.key as String] = kv.value as String;
+        }
+        translations[entry.key as String] = localeDict;
+      }
+    }
+
     final defaultLocale = (map['defaultLocale'] as String?) ?? 'en';
     final supportedLocales = _stringList(
       map,
@@ -95,6 +120,7 @@ class GamePackageManifest {
       display: display,
       resources: resources,
       modules: modules,
+      translations: translations,
       base: base,
     );
   }