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)

docs/rewrite/KNOWN_DIFFERENCES.md

@@ -11,6 +11,7 @@ What remains is verification coverage that this Linux machine cannot provide:
 - Native Apple execution: Metal, IOSurface, CoreText, Apple input APIs, Mach handling and Apple packaging (compile-checked only, `just check-macos`).
 - Native Windows (real Windows console host, Windows-only APIs Wine stubs: `ResizePseudoConsole` is a Wine stub, POSIX-semantics rename over an existing file is unsupported by Wine). The Windows code paths are otherwise run under Wine (`just test-windows-wine`, see `notes/windows-wine.md`): ConPTY, `CreateProcessW`, pipes, the IO thread and `cmd.exe` children are runtime-tested; `just check-windows` stays the clippy gate.
 - Native FreeBSD execution (cross `cargo check` only, if the target could be added).
-- Live portal / D-Bus integration.
+- Real input methods (ibus/fcitx are not installed; the IME paths are covered with a mock `GtkIMContext`, see `notes/verification-VR.md`).
+- Desktop-specific portal backends (GNOME/KDE GlobalShortcuts, a real notification daemon). The D-Bus/portal protocol Ghostty speaks (GTK single-instance and `+new-window`, desktop notifications, GlobalShortcuts, Settings colour scheme, systemd cgroup scope, xdg-open) *is* now covered at runtime against the original on a private bus, with mocks and the real xdg-desktop-portal + gtk backend (`just live-dbus`, `notes/live-dbus.md`).
 - Real-browser rendering (wasm is checked with Node only).
 - Native io_uring execution; Swift code was excluded.