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/1.2/search-overlay.blp

@@ -0,0 +1,94 @@
+using Gtk 4.0;
+using Gdk 4.0;
+using Adw 1;
+
+template $GhosttySearchOverlay: Adw.Bin {
+  visible: bind template.active;
+  halign-target: end;
+  valign-target: start;
+  halign: bind template.halign-target;
+  valign: bind template.valign-target;
+
+  GestureDrag {
+    button: 1;
+    propagation-phase: capture;
+    drag-end => $on_drag_end();
+  }
+
+  Adw.Bin {
+    Box container {
+      styles [
+        "background",
+        "search-overlay",
+      ]
+
+      orientation: horizontal;
+      spacing: 6;
+
+      SearchEntry search_entry {
+        placeholder-text: _("Find…");
+        width-chars: 20;
+        hexpand: true;
+        stop-search => $stop_search();
+        search-changed => $search_changed();
+        next-match => $next_match();
+        previous-match => $previous_match();
+
+        EventControllerKey {
+          // We need this so we capture before the SearchEntry.
+          propagation-phase: capture;
+          key-pressed => $search_entry_key_pressed();
+        }
+      }
+
+      Label {
+        styles [
+          "dim-label",
+        ]
+
+        label: bind $match_label_closure(template.has-search-selected, template.search-selected, template.has-search-total, template.search-total) as <string>;
+        width-chars: 6;
+        xalign: 1.0;
+      }
+
+      Box button_box {
+        orientation: horizontal;
+        spacing: 1;
+
+        styles [
+          "linked",
+        ]
+
+        Button prev_button {
+          icon-name: "go-up-symbolic";
+          tooltip-text: _("Previous Match");
+          clicked => $next_match();
+
+          cursor: Gdk.Cursor {
+            name: "pointer";
+          };
+        }
+
+        Button next_button {
+          icon-name: "go-down-symbolic";
+          tooltip-text: _("Next Match");
+          clicked => $previous_match();
+
+          cursor: Gdk.Cursor {
+            name: "pointer";
+          };
+        }
+      }
+
+      Button close_button {
+        icon-name: "window-close-symbolic";
+        tooltip-text: _("Close");
+        clicked => $stop_search();
+
+        cursor: Gdk.Cursor {
+          name: "pointer";
+        };
+      }
+    }
+  }
+}