Fixes#2700 (with #2704)
A restart (`restart` console command, `AutoRestart`, `CrashGuard`) launches a copy of the server and exits. That copy is detached from the console it was started from: the shell takes the terminal back and console commands stop working. Under systemd or in a container it's killed as soon as the original process exits.
- New setting `server.restartExitCode` (off by default). When it's set, a restart saves and shuts down as usual and then exits with that code, so a shell loop, systemd unit or container restart policy can start the server again on the same console. Values outside 1–255 log a warning and fall back to the current behaviour.
- Fixes `CrashGuard` crash recovery never restarting the server. Since #1809, `Kill()` only records the request, and the unhandled-exception path called `DoKill()` without it.
- Documents systemd, Docker and shell-loop setups in `dev-docs/server-requirements.md`.
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