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/resources/ui/1.3/debug-warning.blp

@@ -0,0 +1,9 @@
+using Gtk 4.0;
+using Adw 1;
+
+template $GhosttyDebugWarning: Adw.Bin {
+  Adw.Banner {
+    title: _("⚠️ You're running a debug build of Ghostty! Performance will be degraded.");
+    revealed: true;
+  }
+}

crates/ghostty-apprt-gtk/resources/ui/1.3/surface-child-exited.blp

@@ -0,0 +1,22 @@
+using Gtk 4.0;
+using Adw 1;
+
+template $GhosttySurfaceChildExited: Adw.Bin {
+  styles [
+    "child-exited",
+  ]
+
+  notify::data => $notify_data();
+
+  Adw.Bin {
+    Adw.Banner banner {
+      button-clicked => $clicked();
+      revealed: true;
+      // Not localized on purpose because it should never be seen.
+      title: "This is a bug in Ghostty. Please report it.";
+      button-label: _("Close");
+      halign: fill;
+      valign: end;
+    }
+  }
+}