Linux debug: hot restart no longer crashes with live mpv players

Hot restart frees the old isolate's NativeCallables while mpv handles stay
alive; media_kit then quits them on the next start and mpv calls the dead
wakeup callback (SIGSEGV on the mpv 'core' thread). A debug-only runner
hook clears every live handle's wakeup callback on the engine's
on-pre-engine-restart signal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/linux/runner/my_application.cc

@@ -6,6 +6,7 @@
 #endif
 
 #include "flutter/generated_plugin_registrant.h"
+#include "hot_restart_guard.h"
 
 struct _MyApplication {
   GtkApplication parent_instance;
@@ -87,6 +88,7 @@ static void my_application_activate(GApplication* application) {
   gtk_widget_realize(GTK_WIDGET(view));
 
   fl_register_plugins(FL_PLUGIN_REGISTRY(view));
+  hot_restart_guard_attach(view);
 
   gtk_widget_grab_focus(GTK_WIDGET(view));
 }