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

docs/lua-package-format.md

@@ -64,6 +64,53 @@ runtime:*.lua
 
 `runtime:` paths must not contain `/`, `..`, or an empty filename.
 
+## Base packages
+
+A game manifest can declare a `base` field to indicate it depends on a framework package:
+
+```json
+{
+  "gameId": "ludo",
+  "base": "_framework",
+  "modules": { ... }
+}
+```
+
+The `base` field is metadata. Actual loading is controlled by `RuntimeOptions.basePackages`:
+
+```dart
+LuaGameWidget(
+  gameId: 'ludo',
+  runtimeOptions: const RuntimeOptions(
+    basePackages: ['_framework'],
+  ),
+)
+```
+
+Loading order:
+
+1. Base packages are loaded first, in `basePackages` order.
+2. The game package is loaded last.
+3. All modules are merged into a flat map.
+4. Later packages override earlier packages on name collision.
+5. The entry script always comes from the last (game) package.
+
+This means a game can override any framework module by declaring a module with the same name in its own manifest.
+
+## Multi-package module resolution
+
+When Lua code calls `runtime.import("xxx")`:
+
+1. Look up `xxx` in the merged module map (game modules first, then framework).
+2. If not found, throw `FormatException: Lua module is not declared in manifest.modules`.
+
+Framework modules are transparent to game code:
+
+```lua
+local app = runtime.import("app")       -- resolved from framework
+local state = runtime.import("state")   -- resolved from game
+```
+
 ## Entry module
 
 The manifest `entry` module should expose lifecycle/event functions expected by the script engine. Keep game-specific state in Lua modules and return runtime diffs/commands through the approved protocol.