event_loop: client binding for ext-foreign-toplevel-list-v1

Adds an `App` callback that delivers the live list of open toplevels from the compositor — the data source a shell needs for dock running-app indicators, taskbar tiles, alt-tab and any other "what is currently running" UI. Hand-wiring the protocol binding from every shell that wants it is the kind of boilerplate ltk should absorb once: this is that move.
`Cargo.toml` adds `"staging"` to `wayland-protocols`' feature list. SCTK 0.20 already pulls staging in transitively (it carries `foreign_toplevel_list.rs` and its own dispatch helper), so this is belt-and-braces against a future ltk tree that swaps SCTK for a different client toolkit — it keeps the protocol available crate-wide even then. `src/app.rs` introduces `ToplevelEvent { Opened { id: u32, app_id: String }, Closed { id: u32 } }` and `App::on_toplevel_event( &self, ToplevelEvent ) -> Option<Self::Message>` with the default returning `None` so apps that do not care pay nothing (no allocation, no dispatch). `id` is the Wayland protocol id of the handle proxy — unique per session, stable for the handle's lifetime, the same value paired across `Opened` and `Closed`. `src/lib.rs` re-exports `ToplevelEvent` from the public prelude.
`src/event_loop/app_data.rs` grows a `pub foreign_toplevel_list: ForeignToplevelList` field. `src/event_loop/mod.rs` constructs it via `ForeignToplevelList::new( &globals, &qh )` and stores it on `AppData`. If the compositor does not advertise the global the inner `GlobalProxy` just resolves to "absent" and the list yields no toplevels — no error path needed at construction. `src/event_loop/handlers.rs` adds the `Proxy` import, `delegate_foreign_toplevel_list!( @<A: App> AppData<A> )` next to the rest, and implements `ForeignToplevelListHandler` for `AppData<A>`. The three SCTK callbacks (`new_toplevel`, `update_toplevel`, `toplevel_closed`) each pull the handle's protocol id and its currently-cached `app_id` from the list's info cache, call `self.app.on_toplevel_event( … )`, and push the returned message onto `pending_msgs` so it flows through the normal `update` cycle with the regular `invalidate_after` scoping path. `update_toplevel` re-emits `Opened` with the latest info — compositors fire this on title changes too, not just `app_id` changes, but apps whose state is keyed on `(id, app_id)` can absorb the repeat idempotently and apps that need title-change granularity can scope via `invalidate_after`.
The wire-up is generic: a shell that wants finer behaviour (focus follow, per-title indicators, multi-window grouping) can layer on top by translating the event into more specific app messages. The default app pays zero, the shell that opts in gets a real event stream without touching `smithay-client-toolkit` directly.

src/event_loop/app_data.rs

@@ -619,6 +619,16 @@ pub struct AppData<A: App>
 	pub current_cursor_shape: Option<crate::types::CursorShape>,
 	pub text_input_manager: Option<ZwpTextInputManagerV3>,
 	pub text_input:         Option<ZwpTextInputV3>,
+	/// Client-side handle to `ext-foreign-toplevel-list-v1`. Holds the
+	/// global proxy plus the live list of open toplevels; SCTK fans
+	/// events through this object into our `ForeignToplevelListHandler`
+	/// impl in `handlers.rs`, which then routes them to
+	/// `App::on_toplevel_event`. Present on every session, even when
+	/// the compositor does not advertise the global — internally the
+	/// list holds a `GlobalProxy` that simply yields no toplevels in
+	/// that case.
+	pub foreign_toplevel_list:
+		smithay_client_toolkit::foreign_toplevel_list::ForeignToplevelList,
 	pub shift_pressed:      bool,
 	pub ctrl_pressed:       bool,
 	/// Calloop handle for inserting timers / channels. Used by the