test(gtk): live D-Bus/portal comparison against the original Ghostty

Adds scripts/live-dbus-test.sh (`just live-dbus`): each scenario runs
the installed Ghostty 1.3.1 and the Rust build on a private
dbus-run-session bus inside a private headless weston (the user's
session bus is never written to), with Python/Gio mocks for
org.freedesktop.Notifications, the GlobalShortcuts/Settings portal and
org.freedesktop.systemd1, plus a real xdg-desktop-portal run. Scenarios:
single-instance (+detect), +new-window with arguments, notifications,
global shortcuts, color-scheme settings, cgroup scope creation, URL
opening. tests/live_dbus.rs compares the Rust build against goldens
recorded from the original (skips when tools or a fresh production
build are missing).

Two Rust bugs found and fixed (no upstream deviation):
- GlobalShortcuts CreateSession/BindShortcuts payloads used the wrong
  D-Bus signatures ((v), double-boxed values), which a real portal
  rejects; signature tests added.
- CreateSession was sent twice at startup; the config is now bound only
  after the D-Bus connection is set, as in Zig.

All nine scenarios now behave identically to the original
(docs/rewrite/notes/live-dbus.md).

Test Plan:
- just live-dbus
- cargo test --test live_dbus (needs target/vp/production/ghostty)

crates/ghostty-apprt-gtk/src/class/application.rs

@@ -293,6 +293,12 @@ mod imp {
         /// The global shortcut logic.
         pub global_shortcuts: OnceCell<GlobalShortcuts>,
 
+        /// True once the shortcuts' `config` property is bound to ours
+        /// (Zig: `bindProperty` in `startupGlobalShortcuts`, after the
+        /// D-Bus connection was set; before that the config must not reach
+        /// the shortcuts, or the portal session is created twice).
+        pub global_shortcuts_bound: Cell<bool>,
+
         /// Set to true so long as we request a window exactly once. This
         /// prevents quitting the app before we've shown one window.
         pub requested_window: Cell<bool>,
@@ -827,7 +833,9 @@ impl Application {
         }
 
         // Keep global shortcuts in sync with the config (property binding).
-        if let Some(gs) = self.imp().global_shortcuts.get() {
+        if self.imp().global_shortcuts_bound.get()
+            && let Some(gs) = self.imp().global_shortcuts.get()
+        {
             gs.set_config(self.imp().config.borrow().as_ref());
         }
     }
@@ -1075,6 +1083,7 @@ impl Application {
         gs.set_connection(self.dbus_connection().as_ref());
 
         // Keep the shortcut config in sync with the app.
+        self.imp().global_shortcuts_bound.set(true);
         gs.set_config(self.imp().config.borrow().as_ref());
 
         // Setup the signal handler for global shortcut triggers