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/1.0/clipboard-confirmation-dialog.blp

@@ -0,0 +1,86 @@
+using Gtk 4.0;
+// This is unused but if we remove it we get a blueprint-compiler error.
+using Adw 1;
+
+template $GhosttyClipboardConfirmationDialog: $GhosttyDialog {
+  styles [
+    "clipboard-confirmation-dialog",
+  ]
+
+  notify::blur => $notify_blur();
+  notify::request => $notify_request();
+  heading: _("Authorize Clipboard Access");
+  // Not localized because this is a placeholder users never see.
+  body: "If you see this text, there is a bug in Ghostty. Please report it.";
+
+  responses [
+    cancel: _("Deny") suggested,
+    ok: _("Allow") destructive,
+  ]
+
+  default-response: "cancel";
+  close-response: "cancel";
+
+  extra-child: ListBox {
+    selection-mode: none;
+
+    Overlay {
+      styles [
+        "osd",
+        "clipboard-overlay",
+      ]
+
+      ScrolledWindow text_view_scroll {
+        width-request: 500;
+        height-request: 200;
+
+        TextView text_view {
+          styles [
+            "clipboard-contents",
+          ]
+
+          cursor-visible: false;
+          editable: false;
+          monospace: true;
+          top-margin: 8;
+          left-margin: 8;
+          bottom-margin: 8;
+          right-margin: 8;
+          buffer: bind template.clipboard-contents;
+        }
+      }
+
+      [overlay]
+      Button reveal_button {
+        clicked => $reveal_clicked();
+        visible: false;
+        halign: end;
+        valign: start;
+        margin-end: 12;
+        margin-top: 12;
+
+        Image {
+          icon-name: "view-reveal-symbolic";
+        }
+      }
+
+      [overlay]
+      Button hide_button {
+        clicked => $hide_clicked();
+        visible: false;
+        halign: end;
+        valign: start;
+        margin-end: 12;
+        margin-top: 12;
+
+        styles [
+          "opaque",
+        ]
+
+        Image {
+          icon-name: "view-conceal-symbolic";
+        }
+      }
+    }
+  };
+}