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)

tests/live_dbus.rs

@@ -0,0 +1,89 @@
+#![allow(clippy::unwrap_used)]
+//! Runs `scripts/live-dbus-test.sh` in golden mode: the Rust build is started
+//! on a private D-Bus session bus inside a private headless weston, with mock
+//! Notifications / systemd1 / portal services (and the real
+//! xdg-desktop-portal + gtk backend), and its observable D-Bus behaviour is
+//! compared with `scripts/live-dbus/golden/*.txt`, recorded from the original
+//! Ghostty 1.3.1 (`UPDATE_GOLDEN=1 just live-dbus` re-records them against the
+//! installed original). See `docs/rewrite/notes/live-dbus.md`.
+//!
+//! Skipped (passes) when a needed tool is missing, when there is no
+//! release-profile Rust build to test (debug builds use a different D-Bus
+//! application id, `com.mitchellh.ghostty-debug`) or when that build is older
+//! than the sources (build it with `scripts/live-dbus/build-rust.sh`). Set
+//! `GHOSTTY_SKIP_LIVE_DBUS` to skip explicitly.
+
+use std::{
+    path::{Path, PathBuf},
+    process::Command,
+    time::SystemTime,
+};
+
+fn newest_source(dir: &Path, newest: &mut SystemTime) {
+    let Ok(entries) = std::fs::read_dir(dir) else {
+        return;
+    };
+    for e in entries.flatten() {
+        let p = e.path();
+        let name = e.file_name();
+        if p.is_dir() {
+            if name != "target" && name != ".git" {
+                newest_source(&p, newest);
+            }
+        } else if matches!(
+            p.extension().and_then(|x| x.to_str()),
+            Some("rs" | "toml" | "blp" | "ui" | "css")
+        ) && let Ok(m) = e.metadata().and_then(|m| m.modified())
+        {
+            *newest = (*newest).max(m);
+        }
+    }
+}
+
+fn have(tool: &str) -> bool {
+    Command::new("sh")
+        .args(["-c", &format!("command -v {tool}")])
+        .output()
+        .is_ok_and(|o| o.status.success())
+}
+
+#[test]
+fn rust_build_matches_recorded_original_on_private_bus() {
+    let root = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    if std::env::var_os("GHOSTTY_SKIP_LIVE_DBUS").is_some() {
+        return;
+    }
+    for tool in ["weston", "dbus-run-session", "busctl", "python3", "xdotool"] {
+        if !have(tool) {
+            eprintln!("live-dbus: skipped ({tool} missing)");
+            return;
+        }
+    }
+    let bin = std::env::var_os("RUST_BIN")
+        .map_or_else(|| root.join("target/vp/production/ghostty"), PathBuf::from);
+    let Ok(built) = std::fs::metadata(&bin).and_then(|m| m.modified()) else {
+        eprintln!("live-dbus: skipped (no release build at {})", bin.display());
+        return;
+    };
+    let mut newest = SystemTime::UNIX_EPOCH;
+    for d in ["crates", "src", "resources"] {
+        newest_source(&root.join(d), &mut newest);
+    }
+    if newest > built {
+        eprintln!(
+            "live-dbus: skipped ({} is older than the sources)",
+            bin.display()
+        );
+        return;
+    }
+    let out = Command::new("bash")
+        .arg(root.join("scripts/live-dbus-test.sh"))
+        .env("LABELS", "rust")
+        .env("RUST_BIN", &bin)
+        .env("OUT", root.join("target/live-dbus"))
+        .output()
+        .unwrap();
+    let text = String::from_utf8_lossy(&out.stdout);
+    eprintln!("{text}");
+    assert!(out.status.success(), "live-dbus comparison failed:\n{text}");
+}