fix: address codex review findings on probe/watchdog fallback

- a probe or raster-stall failure record now also downgrades sessions
  that are already running: main_set_local_option broadcasts the
  fallback for failed-* health writes, which also closes the hole where
  the watchdog's idempotence guard no-oped after the probe had already
  written the record
- probe success requires a frame timing newer than the first push:
  'consumed' advances inside the plugin callback before the GL/Metal
  upload, so a raster thread hanging in the driver no longer counts as
  a pass (and cannot clear a previous failure)
- watchdog failures are tagged with the failing backend
  (failed-watchdog-rgba/gpu); the rgba-only probe clears only the rgba
  class, so a working pixel-buffer path can no longer re-enable a
  broken D3D shared-handle path every launch
- the watchdog pauses while the session's window is hidden (new
  session_set_render_visible FFI wired to the window minimize/restore
  events): a display registered in a minimized window no longer records
  a false global failure; observation now counts pushes within the
  window rather than since registration
- probe failure verdicts additionally require a resumed lifecycle,
  matching the raster-stall monitor
- linux plugin: deferred-unref grace lengthened to 10s (no raster-side
  completion barrier exists; documented as heuristic), ref bumped

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

flutter/lib/web/bridge.dart

@@ -1462,6 +1462,9 @@ class RustdeskImpl {
       required int ptr,
       dynamic hint}) {}
 
+  void sessionSetRenderVisible(
+      {required UuidValue sessionId, required bool visible, dynamic hint}) {}
+
   bool mainTextureRenderProbeSupported({dynamic hint}) {
     return false;
   }