build(mobile): declare App platform compiler to stop build:app false green

`@dcloudio/uni-app-plus` was never declared, so `uni build -p app` silently
fell back to an H5 shell: exit code 0, `DONE Build complete.`, but the output
was only `index.html` (still carrying the `/h5/assets/...` base path) with no
`app-service.js`, `app-renderjs.js` or `manifest.json`. Because
`docs/MOBILE_APP_PACKAGING.md` lists that command as a local verification
step, App platform code was reported as built without ever reaching the App
compiler.

Declare the compiler at the same version as the other `@dcloudio` packages,
guard the version alignment in `tests/app-packaging.test.mjs`, and document
that success is judged by the emitted artifacts, not by the DONE line.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/MOBILE_APP_PACKAGING.md

@@ -35,6 +35,7 @@ npm --prefix mobile-uni run build:app
 - 现有品牌源图仅为 64×64，因此这里生成的是**可用于自定义基座和测试包的派生资产**。正式提交商店前应替换为品牌方提供的高分辨率母版，再重新执行生成和预检。
 - `androidPrivacy.json.example` 是**未启用的法务模板**，其中 `__TERMS_URL__` 与 `__PRIVACY_URL__` 必须替换为法务确认的 HTTPS 地址。复制为被 Git 忽略的 `androidPrivacy.json` 后，`npm --prefix mobile-uni run verify:app-release` 才会通过。
 - iOS 已按实际功能填写麦克风和照片库用途说明；不申请相机、定位、通讯录、蓝牙或广告追踪权限。
+- `build:app` 依赖 `@dcloudio/uni-app-plus`（须与其他 `@dcloudio` 包同版本）。缺少该编译器时命令**不报错**，只静默产出带 `/h5/` 基路径的 H5 空壳，产物里没有 `app-service.js` 与 `app-renderjs.js`，App 平台代码等于从未编译。判定 `build:app` 是否真的成功要看产物中是否存在 `app-service.js`、`app-renderjs.js`、`manifest.json`，不能只看 `DONE Build complete.`。
 - App-Plus 当前不启用浏览器 WebRTC 实时对练；界面必须降级为录音后转写，不能将该路径宣传为实时语音通话。
 - 真机步骤和证据口径见 `docs/MOBILE_APP_DEVICE_ACCEPTANCE.md`。资源预检通过不等于 Android/iOS 真机或应用商店验收通过。