feat(practice): run realtime practice on App-Plus through renderjs

App-Plus has no WebRTC or getUserMedia in the logic layer, so realtime
practice was H5-only and the native entry had to advertise a record-then-
transcribe downgrade.

Move the media/WebRTC engine into `realtime-browser-engine.js` and drive it
from `RealtimePracticeAppBridge.vue`, whose renderjs script runs inside the
system WebView where those APIs do exist. The logic layer keeps the
authenticated calls: SDP exchange and `search_knowledge` tool invocation stay
server-proxied, so the access token is never handed to the view layer. Both
sides talk over the renderjs bridge only in session payloads the server
already assembled.

Verified against a real App build (`app-renderjs.js` holds RTCPeerConnection
and getUserMedia; `app-service.js` holds the SDP endpoint, renderjs side has
zero hits). Microphone permission, live transcription, remote playback,
foreground/background switching and disconnect fallback still require Android
and iOS device verification.

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

docs/MOBILE_APP_PACKAGING.md

@@ -36,7 +36,7 @@ npm --prefix mobile-uni run build:app
 - `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。
+- App-Plus 实时对练通过 `renderjs` 在系统 WebView 中执行媒体/WebRTC；认证 SDP 与工具请求仍由逻辑层代理。发布前必须在 Android/iOS 真机验证麦克风授权、实时转写、远端语音播放、前后台切换和断线降级。
 - 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。
 
 ## 更新策略