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/scenario.sh

@@ -0,0 +1,89 @@
+#!/usr/bin/env bash
+# One scenario against one binary, executed inside `dbus-run-session`
+# (see ../live-dbus-test.sh).  usage: scenario.sh <scenario> <bin> <outdir>
+# Writes <outdir>/{bus.json,mock.jsonl,facts.txt,...}; normalize.py turns them
+# into the comparable result.txt.
+set -u
+SCEN=$1; BIN=$2; D=$3
+W=$LDB_RUN
+LD=$LDB_ROOT/scripts/live-dbus
+mkdir -p "$W/home" "$W/cfg/ghostty" "$W/data" "$W/cache"
+export HOME=$W/home XDG_CONFIG_HOME=$W/cfg XDG_DATA_HOME=$W/data XDG_CACHE_HOME=$W/cache
+export GSETTINGS_BACKEND=memory GDK_BACKEND=wayland LANG=C.UTF-8 SHELL=/bin/sh
+export MOCK_LOG=$D/mock.jsonl
+unset TERM_PROGRAM TERM_PROGRAM_VERSION GHOSTTY_BIN_DIR GHOSTTY_SHELL_FEATURES
+export GHOSTTY_RESOURCES_DIR=${GHOSTTY_RESOURCES_DIR:-}
+[ -z "$GHOSTTY_RESOURCES_DIR" ] && unset GHOSTTY_RESOURCES_DIR
+FACTS=$D/facts.txt; : >"$FACTS"
+fact() { echo "$*" >>"$FACTS"; }
+PIDS=()
+bg() { "$@" & PIDS+=($!); }
+finish() {
+  for p in "${PIDS[@]}"; do kill "$p" 2>/dev/null; done
+  for p in /proc/[0-9]*; do
+    grep -qa "LDB_TAG=$LDB_TAG" "$p/environ" 2>/dev/null || continue
+    case "$(cat "$p/comm" 2>/dev/null)" in dbus-*|bash|timeout) continue ;; esac
+    kill "${p#/proc/}" 2>/dev/null
+  done
+  sleep 0.5
+}
+trap finish EXIT
+
+cfg() { printf '%s\n' "shell-integration = none" "confirm-close-surface = false" \
+  "quit-after-last-window-closed = false" "window-theme = auto" "$@" >"$W/cfg/ghostty/config"; }
+
+start_bus_tools() { # [mock names]
+  busctl --user monitor --json=short >"$D/bus.json" 2>"$D/monitor.err" &
+  PIDS+=($!)
+  MOCK_NAMES=${1:-notifications,systemd,portal} python3 -W ignore "$LD/mock.py" >"$D/mock.out" 2>&1 &
+  PIDS+=($!)
+  for _ in $(seq 50); do grep -q READY "$D/mock.out" && break; sleep 0.1; done
+  sleep 0.5
+}
+
+# Wait until the app owns its bus name.
+wait_name() { for _ in $(seq 300); do busctl --user list --acquired --no-pager 2>/dev/null | grep -q "^$1 " && return 0; sleep 0.1; done; return 1; }
+mockctl() { gdbus call --session --dest test.ghostty.Mock --object-path /test/ghostty/Mock --method test.ghostty.Mock."$@" >/dev/null; }
+
+# GUI window objects exported on the bus by GtkApplication.
+windows() { # [bus name]
+  # (never activate the service: only query a name that is already owned)
+  busctl --user list --acquired --no-pager 2>/dev/null | grep -q "^${1:-com.mitchellh.ghostty} " || { echo 0; return; }
+  busctl --user tree "${1:-com.mitchellh.ghostty}" --list --no-pager 2>/dev/null | grep -c '/window/[0-9]*$'
+}
+# Number of GUI instances of the binary under test (not +actions).
+instances() {
+  local n=0 p
+  for p in /proc/[0-9]*; do
+    [ "$(readlink "$p/exe" 2>/dev/null)" = "$(readlink -f "$BIN")" ] || continue
+    tr '\0' ' ' <"$p/cmdline" | grep -q ' +' && continue
+    n=$((n+1))
+  done
+  echo $n
+}
+# Command lines of terminal children (sh/sleep/...) of GUI instances, sorted.
+children() {
+  local p c
+  for p in /proc/[0-9]*; do
+    [ "$(readlink "$p/exe" 2>/dev/null)" = "$(readlink -f "$BIN")" ] || continue
+    tr '\0' ' ' <"$p/cmdline" | grep -q ' +' && continue
+    for c in $(pgrep -P "${p#/proc/}"); do
+      echo "$(tr '\0' ' ' <"/proc/$c/cmdline") cwd=$(readlink /proc/$c/cwd)"
+    done
+  done | sort
+}
+# pids of processes with this comm that belong to this run
+tagged() { local p; for p in /proc/[0-9]*; do
+  [ "$(cat "$p/comm" 2>/dev/null)" = "$1" ] && grep -qa "LDB_TAG=$LDB_TAG" "$p/environ" 2>/dev/null && echo "${p#/proc/}"
+done; }
+settle() { sleep "${1:-2}"; }
+
+run_ipc() { # label args...
+  local label=$1; shift
+  timeout 15 "$BIN" "$@" >"$D/ipc-$label.log" 2>&1
+  fact "ipc $label rc=$?"
+  grep -E "D-Bus|not valid|error" "$D/ipc-$label.log" | grep -v "^warning\|^info" | sed 's/^/  ipc-err: /' >>"$FACTS"
+}
+
+. "$LD/scenarios.sh"
+"scn_${SCEN//-/_}"