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)

justfile

@@ -299,3 +299,10 @@ check-all: clippy test check-wasm check-windows
 # Capture original Ghostty vs the Rust build in a headless weston and diff them.
 visual-parity *scenarios:
     RUST_BIN="${RUST_BIN:-target/vp/production/ghostty}" scripts/visual-parity.sh {{scenarios}}
+
+# Live D-Bus / portal comparison (private bus + nested weston): notifications,
+# single-instance, global shortcuts, color-scheme, cgroup scope. Builds the
+# iteration binary first. UPDATE_GOLDEN=1 re-records scripts/live-dbus/golden.
+live-dbus *scenarios:
+    scripts/live-dbus/build-rust.sh
+    RUST_BIN="${RUST_BIN:-target/vp/production/ghostty}" scripts/live-dbus-test.sh {{scenarios}}