Layer::Window overlays: an OverlaySpec can now be an ordinary xdg toplevel, named through App::overlay_title; showcase persists the last pressed button too

A shell built on ltk had no way to show something as a regular window: every overlay is a layer-shell surface, and layer-shell surfaces are either below all application windows or above all of them. crustace's "closing applications…" card ran into exactly that — on the overlay layer it covered the applications' own "save changes?" prompts, on the bottom layer it vanished behind their windows — and what that card wants is to be one window among the others: decorated by the compositor, stackable, listed in the switcher and the dock.
`Layer` gains a `Window` variant. An `OverlaySpec` carrying it is materialised by the reconciler as an `xdg_toplevel` (sctk `Window`, server-side decorations) instead of a layer surface: `size` is its fixed size, applied through min/max size (a zero component falls back to a default, since a toplevel cannot "fill"); anchor, exclusive zone and keyboard exclusivity are ignored. The window's app_id is `App::app_id()` and its title comes from the new, defaulted `App::overlay_title( id )`, so the compositor's window lists show something meaningful. `WindowHandler::configure` and `request_close` now resolve which surface a window belongs to: the main window keeps its behaviour, an overlay window is configured through its own `SurfaceState` (with the window geometry set to the configured size) and the compositor's close button delivers the spec's `on_dismiss` instead of exiting the app. Layer-shell applications now bind `xdg_wm_base` when the compositor offers it, so they can open such a window later. Adding a variant keeps every existing `OverlaySpec` literal compiling; the new trait method has a default.
The showcase example now also saves and restores `last_pressed`, bumping its private state format to version 2 with the note kept last so its line breaks survive.

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 
 - **`xdg-session-management-v1` (client)** — before the first commit of a `ShellMode::Window` toplevel the runtime binds `xdg_session_manager_v1`, issues `get_session( reason, stored_id )` and `restore_toplevel( toplevel, "main" )`, so a supporting compositor restores window geometry on every launch. Bindings are generated in-tree from the vendored XML under `protocols/` with `wayland-scanner` (`src/protocol/`).
 - **Runtime session persistence** — `$XDG_STATE_HOME/<app_id>/session.json` (compositor session id, clean-exit marker, pid) plus `state.bin` (the bytes from `App::save_state`), written atomically with mode `0600`; saved every 30 s when the bytes changed, on close and on signal; handed back through `App::restore_state` before the first frame only on a session restore (`LTK_SESSION_RESTORE=1`) or after an unclean exit. Module `src/session_state.rs`.
+- **`Layer::Window` overlays** — an `OverlaySpec` with `layer: Layer::Window` is materialised as an ordinary `xdg_toplevel` of the given fixed size instead of a layer-shell surface: decorated, stacked and listed by the compositor like an application window. Anchor, exclusive zone and keyboard exclusivity are ignored; the compositor's close button delivers `on_dismiss`. `App::overlay_title( id )` (defaulted to the app id) names it. Layer-shell apps now bind `xdg_wm_base` when available so they can open one.
 - **Clean exit on `SIGTERM` / `SIGINT`** — the runtime installs a calloop signal source and leaves the event loop instead of dying, so `save_state` runs and `ltk::run` returns.
 
 - **`Slider::on_release` / `VSlider::on_release`** — fired once with the final value when the drag ends, so an app can keep an expensive commit (a subprocess, a D-Bus round trip, a compositor reconfigure) off the per-motion `on_change` path and still move the thumb live. The gesture machine emits it from the slider branch of `on_release`; `on_change` alone behaves exactly as before.