fix: texture render lifetime protocol, watchdog fallback, startup probe

Root cause of #15848 (and the long-standing macOS #6296 / Linux #3343
class): raw native texture pointers are shared across the platform
thread, the engine raster thread and the video thread, with teardown
ordered by a 100 ms sleep - or, when moving a tab to a new window, by
nothing at all. A lost race frees the texture while it is still in use:
the raster thread parks on a destroyed lock (frozen/black view, a
never-presented 'transparent' hole, every later session black) and the
video thread hangs while holding session locks (app half-dead until
restart, still reported as Responding).

- unregister textures with compare-and-clear (new session_unregister_*
  FFI): a late clear can no longer wipe a new window's registration
  (#8016) and Rust never keeps pushing into a freed texture; the 100 ms
  sleeps are gone (the plugins now drain in-flight pushes and defer
  object deletion until the raster thread is done)
- guard the async texture create path against destroy racing it (#13596)
- per-display locks: the per-frame plugin call no longer holds
  session-level locks, so a stalled plugin or driver call cannot freeze
  every window's UI thread
- adopt the frame size after 30 consecutive mismatches instead of
  dropping frames forever (silent black screen on a live connection)
- watchdog: frames pushed but never consumed by the engine fall the
  session back to software rendering live, record texture-render-health,
  and flip the effective default off; toggling the option clears the
  record and re-arms validation
- Dart raster-stall monitor records a hung raster thread for the next
  launch (rendering cannot be rescued in-process in that state)
- startup probe: render one frame through a 1x1 texture in the main
  window each launch; failure disables texture rendering before the
  first session goes black, a pass self-heals a stale failure record

Platform defaults are unchanged (macOS off, Win10+ on, Linux on).
Requires flutter_texture_rgba_renderer ad4c37e and
flutter_gpu_texture_renderer 767bb9f (pinned in pubspec).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

flutter/lib/models/model.dart

@@ -735,6 +735,11 @@ class FfiModel with ChangeNotifier {
   _handleUseTextureRender(
       Map<String, dynamic> evt, SessionID sessionId, String peerId) {
     parent.target?.imageModel.setUseTextureRender(evt['v'] == 'Y');
+    if (evt['reason'] == 'fallback') {
+      // The Rust watchdog detected that pushed frames were never rendered
+      // and switched this session to software rendering.
+      showToast(translate('texture-render-fallback-tip'));
+    }
     waitForFirstImage.value = true;
     isRefreshing = true;
     showConnectedWaitingForImage(parent.target!.dialogManager, sessionId,