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)

scripts/live-dbus/child-theme.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+# Enables color-scheme reporting (mode 2031), asks for the current scheme
+# (CSI ? 996 n) and keeps asking for the background colour (OSC 11).
+esc=$(printf '\033'); bel=$(printf '\007')
+stty raw -echo
+printf '%s[?2031h%s[?996n' "$esc" "$esc"
+( while :; do printf '%s]11;?%s' "$esc" "$bel"; sleep 0.7; done ) &
+exec cat -v >>"$OUTF"