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/pubspec.lock

@@ -538,8 +538,8 @@ packages:
     dependency: "direct main"
     description:
       path: "."
-      ref: "08a471bb8ceccdd50483c81cdfa8b81b07b14b87"
-      resolved-ref: "08a471bb8ceccdd50483c81cdfa8b81b07b14b87"
+      ref: "767bb9fe9dcd2c23e2664114bf33760842e872e7"
+      resolved-ref: "767bb9fe9dcd2c23e2664114bf33760842e872e7"
       url: "https://github.com/rustdesk-org/flutter_gpu_texture_renderer"
     source: git
     version: "0.0.1"
@@ -1298,8 +1298,8 @@ packages:
     dependency: "direct main"
     description:
       path: "."
-      ref: "42797e0f03141dc2b585f76c64a13974508058b4"
-      resolved-ref: "42797e0f03141dc2b585f76c64a13974508058b4"
+      ref: "ad4c37e414ee40853c0e9503b2da6486d4c1768d"
+      resolved-ref: "ad4c37e414ee40853c0e9503b2da6486d4c1768d"
       url: "https://github.com/rustdesk-org/flutter_texture_rgba_renderer"
     source: git
     version: "0.0.16"