feat(apprt-gtk): port the GTK4/libadwaita app runtime

Ports ZIG/src/apprt/gtk into ghostty-apprt-gtk: every GObject class
(application, window, tab, split tree, surface + scrolled window,
child-exited banner, search/key-state/resize overlays, command palette,
global shortcuts, all dialogs, config wrapper) as glib subclasses with
the same GType names, properties, signals and template children; GDK
key translation; X11 and Wayland window-protocol helpers (blur, CSD,
quick terminal via gtk4-layer-shell); single-instance D-Bus IPC;
gsettings; cgroup scope creation.

build.rs compiles Ghostty's blueprints with blueprint-compiler and
bundles UI, CSS and icons into a gresource, as Ghostty's build does.

The core is not ported yet, so the crate defines the boundary it needs
in src/core.rs (CoreApp, CoreSurface, RtApp, RtSurface, Waker); the
spec is in docs/rewrite/notes/ghostty-apprt-gtk.md. ghostty-core will
implement these traits (the crate no longer depends on core to avoid a
cycle). examples/demo.rs runs the app with a dummy core that clears the
GLArea; it launches on Wayland without GTK criticals.

Stubs/open items: toggle_visibility, float_window, secure_input and
close_all_windows return false; inspector is a no-op; X11/Wayland
protocol code is untested against real compositors; gtk4-rs doesn't
expose the GLArea framebuffer id, so the renderer must query it.

Also registers ghostty-apprt and ghostty-cli in workspace.dependencies.

Test Plan:
- cargo test -p ghostty-apprt-gtk (36 tests)
- DEMO_QUIT_AFTER_MS=3000 cargo run -p ghostty-apprt-gtk --example demo

crates/ghostty-apprt-gtk/src/weak_ref.rs

@@ -0,0 +1,40 @@
+//! Weak object references (port of `apprt/gtk/weak_ref.zig`). glib-rs
+//! already provides `glib::WeakRef<T>`; this wrapper keeps the Zig API
+//! (`set`/`get`/`empty`).
+
+use gtk::glib;
+use gtk::glib::prelude::*;
+
+/// A weak reference to a GObject of type `T`.
+#[derive(Debug, Default)]
+pub struct WeakRef<T: ObjectType>(glib::WeakRef<T>);
+
+impl<T: ObjectType> WeakRef<T> {
+    /// An empty weak reference.
+    pub fn empty() -> Self {
+        Self(glib::WeakRef::new())
+    }
+
+    pub fn set(&self, v: Option<&T>) {
+        self.0.set(v);
+    }
+
+    pub fn get(&self) -> Option<T> {
+        self.0.upgrade()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn weak_ref() {
+        let r: WeakRef<glib::Object> = WeakRef::empty();
+        let obj = glib::Object::new::<glib::Object>();
+        r.set(Some(&obj));
+        assert!(r.get().is_some());
+        drop(obj);
+        assert!(r.get().is_none());
+    }
+}