app, event_loop: first-frame-committed hook and foreign_toplevel app_id only

Two independent changes, both blockers for a working desktop session through the loginmanager-daemon handoff and the dock's running-app icons.
`App::on_first_frame_committed` is a new trait hook fired exactly once, immediately after the very first `wl_surface.commit` of a rendered buffer on the main surface. `AppData` grows a `first_frame_committed: bool`, `draw_frame` now returns whether this call performed that first commit, and `try_run` invokes the hook after the borrows held during the draw are released. Used by the loginmanager-daemon-aware crustace path to signal "ready to be presented" back to the daemon as soon as the GPU has the first frame — the actual present can still be deferred under VT switching (no DRM master yet), but the client-side commit is the right edge for handoff.
`toplevel_display_id` in the foreign-toplevel-list handler no longer falls back to `info.title` or `info.identifier` when `info.app_id` is empty. Smithay creates each `ext-foreign-toplevel-list-v1` handle with `app_id = ""` and `init_new_instance` flushes a `done` immediately, so subscribers used to see the protocol-level identifier (a 32-char `Alphanumeric` random token) as the "app id" of every new toplevel — and remained stuck on it whenever the client's real `set_app_id` arrived between done events but the subscribing app's matcher couldn't resolve it to a `.desktop` entry. Returning the raw `app_id` (empty or not) makes that first transient `done` ignorable by the consumer's own empty-string guard; the second `done`, carrying the real app id, is processed normally.

src/event_loop/run.rs

@@ -300,6 +300,7 @@ pub( crate ) fn try_run<A: App>( app: A ) -> Result<(), RunError>
 		cached_overlays: None,
 		view_dirty:      true,
 		overlays_dirty:  true,
+		first_frame_committed: false,
 	};
 
 	// Register a calloop channel so the app can send messages from any thread.
@@ -525,7 +526,11 @@ pub( crate ) fn try_run<A: App>( app: A ) -> Result<(), RunError>
 				data.cached_overlays = Some( specs );
 				data.overlays_dirty  = false;
 			}
-			draw_frame( &mut data );
+			let first_commit_now = draw_frame( &mut data );
+			if first_commit_now
+			{
+				data.app.on_first_frame_committed();
+			}
 
 			// Push the freshly-laid-out widget tree to AccessKit. The
 			// closure only runs when an AT client is actually