The POSIX signal that terminated the process, surfaced on Unix only.
public PosixSignal? Signal { get; }
This is a best-effort heuristic derived from ExitCode
(an ExitCode > 128 is interpreted as termination by signal
ExitCode - 128). It is not authoritative and is null on Windows,
where no POSIX signal terminated the process. The property is intentionally
supported on every platform so Windows-targeted consumers can read it (always
null) without triggering CA1416.