v0.9.4: a turn keeps running when the app is not on screen

A phone freezes an app it cannot see, and the front end had nothing holding
it: a turn that took minutes -- a long shell command, a provider that thinks,
a build -- stopped quietly the moment the user pressed home, and the answer
was only there if the window happened to come back. The engine's own /attach
flow made this visible: an APK turn survived, a pocket did not.

- app/android/src/xyz/whcself/cstagent/CstService.java: a foreground service
  started with an explicit component (nothing outside the app can reach it,
  no intent filter) for as long as a turn runs. Being a foreground service
  makes the process one Android neither freezes nor kills; a partial wake
  lock (30 min ceiling) keeps the CPU awake with the screen off. Its ongoing
  notification says what is running -- "CST-agent is working" plus the first
  line of the prompt -- on the same cst-agent channel the notification_send
  tool uses, and taps back into the app. A phone that refuses the foreground
  start (Android 12+ does, from the background) loses the protection, not the
  turn: the failure is caught and the service stops itself.
- Manifest: FOREGROUND_SERVICE, FOREGROUND_SERVICE_DATA_SYNC (API 34+ wants
  the typed one) and WAKE_LOCK, plus the <service> with
  foregroundServiceType="dataSync". The first build of this failed with
  "Expected '>', but got ' '" -- a manifest comment cannot contain "--".
- Front end: app::holdForeground(text) / app::releaseForeground() in
  ToolProvider.h, one call for every platform -- Android starts and stops the
  service through JNI, a desktop has nothing to hold (its window is the app).
  AgentCore::send() calls it when the turn starts (firstLine() of the prompt
  is what the notification says) and finishTurn() releases it, so the
  notification exists exactly as long as the work does. CST_APP_NO_SERVICE=1
  keeps a test run from touching the phone.
- On the NX789J, with the app sent home and the screen off: the service read
  back isForeground=true with its notification, the wake lock cst-agent:turn
  was taken at the start of the turn and dropped at its end, run_shell
  "sleep 60" finished with the screen dark, and the service was gone after.

Desktop: build clean, --uitest all green, --selftest and cst-app-selftest OK
(the hold is a no-op there, so nothing about the Qt side changed).

Version 0.9.4.

docs/android.md

@@ -187,6 +187,16 @@ have, so the script rewrites the shebang of the build-tools wrappers.
   with Open and Share -- the copy lives next to the session, marked `for_user`
   so no request ever contains it (see `app/README.md`, "Files the agent
   delivers").
+* **A turn survives the app leaving the screen**: `AgentCore::send()` calls
+  `app::holdForeground()`, which starts `CstService`
+  (`app/android/src/xyz/whcself/cstagent/CstService.java`) as a foreground
+  service and takes a partial wake lock; `finishTurn()` stops it again. On the
+  NX789J: with the app sent home and the screen off, the service showed
+  `isForeground=true` with its `CST-agent is working` notification (id 2, the
+  same `cst-agent` channel), the lock `cst-agent:turn` was acquired at the
+  start of the turn and released at its end, `run_shell sleep 60` finished with
+  the screen dark, and the service was gone once the turn was over.
+  `CST_APP_NO_SERVICE=1` keeps a test run from touching the phone.
 * Still to come: the SAF picker for attaching a picture from another app, the
   back gesture, and the icon glyphs -- `ThemeIcon` finds no icon files on the
   phone and draws empty boxes where the desktop shows Breeze icons.