Exited with the given code.

Terminated by a signal (Unix). `None` when the signal number is unavailable.

The process concluded, but its actual exit status could not be observed — a native API failure (e.g. Windows `DuplicateHandle`/`GetExitCodeProcess`), or a POSIX reap race whose winner never reported the real decoded status within a grace period. `Reason` is a short, human-readable explanation for logs/diagnostics. This is deliberately NOT a sentinel for a clean exit — never fabricate `Exited 0` for a process whose outcome genuinely was not seen; use this case instead. Always treated as a failure (`IsAcceptedBy` is always `false`, like `Signalled`/`TimedOut`), and never itself a `-1`/`0` stand-in.

The exit code, or `None` for a signal kill, timeout, or unobserved outcome (never a `-1` sentinel).

The terminating signal, when known.