App: add on_main_frame_committed, called after every main-surface commit

A host that hands part of its content to input-transparent subsurfaces has no way to know when the main frame that replaces that content has actually been sent: `view()` runs when the tree is dirty, but the draw can be deferred by a pending frame callback, so a subsurface dropped on the strength of the rebuild alone can vanish a frame before the main paints its replacement. The new hook fires in `draw_frame` right after the main buffer is rendered and committed, before the subsurfaces are reconciled for the same iteration, so the host can retire a subsurface knowing the main frame behind it is already out. `on_first_frame_committed` keeps its one-shot role.

src/app.rs

@@ -561,6 +561,12 @@ pub trait App: 'static
 	/// presented as soon as my VT becomes active".
 	fn on_first_frame_committed( &mut self ) {}
 
+	/// Called every time a buffer for the main surface has been rendered and
+	/// committed, before the subsurfaces are reconciled for the same
+	/// iteration. Lets a host retire or mount a subsurface in the same flush
+	/// as the main frame it replaces, so the swap lands in one frame.
+	fn on_main_frame_committed( &mut self ) {}
+
 	/// Called when the surface is asked to close (compositor request, titlebar button,
 	/// layer-shell closed event). Return `true` to allow the application to exit,
 	/// or `false` to cancel.

src/event_loop/frame.rs

@@ -75,6 +75,7 @@ pub( crate ) fn draw_frame<A: App>( data: &mut AppData<A> ) -> bool
 			data.first_frame_committed = true;
 			first_commit_now = true;
 		}
+		data.app.on_main_frame_committed();
 	}
 
 	for ( _, ss ) in data.secondaries.iter_mut()