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>

mobile-uni/package.json

@@ -17,6 +17,7 @@
   },
   "dependencies": {
     "@dcloudio/uni-app": "3.0.0-alpha-5020120260710001",
+    "@dcloudio/uni-app-plus": "3.0.0-alpha-5020120260710001",
     "@dcloudio/uni-h5": "3.0.0-alpha-5020120260710001",
     "@dcloudio/uni-ui": "^1.5.12",
     "lxgw-wenkai-tc-webfont": "^1.0.0",