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/1.5/command-palette.blp

@@ -0,0 +1,110 @@
+using Gtk 4.0;
+using Gio 2.0;
+using Adw 1;
+
+Adw.Dialog dialog {
+  content-width: 700;
+  closed => $closed();
+
+  Adw.ToolbarView {
+    top-bar-style: flat;
+
+    [top]
+    Adw.HeaderBar {
+      [title]
+      Gtk.SearchEntry search {
+        hexpand: true;
+        placeholder-text: _("Execute a command…");
+        stop-search => $search_stopped();
+        activate => $search_activated();
+
+        styles [
+          "command-palette-search",
+        ]
+      }
+    }
+
+    Gtk.ScrolledWindow {
+      min-content-height: 300;
+
+      Gtk.ListView view {
+        show-separators: true;
+        single-click-activate: true;
+        activate => $row_activated();
+
+        model: Gtk.SingleSelection model {
+          model: Gtk.FilterListModel {
+            incremental: true;
+
+            filter: Gtk.AnyFilter {
+              Gtk.StringFilter {
+                expression: expr item as <$GhosttyCommand>.title;
+                search: bind search.text;
+              }
+
+              Gtk.StringFilter {
+                expression: expr item as <$GhosttyCommand>.action-key;
+                search: bind search.text;
+              }
+            };
+
+            model: Gio.ListStore source {
+              item-type: typeof<$GhosttyCommand>;
+            };
+          };
+        };
+
+        styles [
+          "rich-list",
+        ]
+
+        factory: Gtk.BuilderListItemFactory {
+          template Gtk.ListItem {
+            child: Gtk.Box {
+              orientation: horizontal;
+              spacing: 10;
+              tooltip-text: bind template.item as <$GhosttyCommand>.description;
+
+              Gtk.Box {
+                orientation: vertical;
+                hexpand: true;
+
+                Gtk.Label {
+                  ellipsize: end;
+                  halign: start;
+                  wrap: false;
+                  single-line-mode: true;
+
+                  styles [
+                    "title",
+                  ]
+
+                  label: bind template.item as <$GhosttyCommand>.title;
+                }
+
+                Gtk.Label {
+                  ellipsize: end;
+                  halign: start;
+                  wrap: false;
+                  single-line-mode: true;
+
+                  styles [
+                    "subtitle",
+                    "monospace",
+                  ]
+
+                  label: bind template.item as <$GhosttyCommand>.action-key;
+                }
+              }
+
+              Gtk.ShortcutLabel {
+                accelerator: bind template.item as <$GhosttyCommand>.action;
+                valign: center;
+              }
+            };
+          }
+        };
+      }
+    }
+  }
+}