Shutdown stalls are hard to debug because journald is torn down mid-way,
so late hangs never reach the journal. `modules/shutdown-debug.nix`
exposes `local.shutdownDebug.*`: verbose systemd logging routed to kmsg
(recoverable from pstore after a forced power-off), an opt-in tty9 debug
shell, and a shortened `DefaultTimeoutStopSec` preset (system + user) so
a hung unit is killed and logged in seconds instead of spinning at 90s.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
daniil-berg
