fix: handle mutation child errors and processes option

[wheesnoza]

This PR proposes a small fix for two related behaviours around mutation parallel execution. I am opening it to confirm whether these behaviours are intentional, but both currently seem to make mutation results less reliable.

--parallel is removed from the initial test run so mutation testing can collect coverage before running mutations itself. --processes, however, is still forwarded to that initial run. When --parallel is removed but --processes remains, the initial run can fail before mutation testing starts.

Mutation child processes also treat every non-successful exit as a tested mutation. That makes assertion failures and runtime errors indistinguishable, so a child process error can be counted as a killed mutation and inflate the score.

What changed

ProcessesOption is now removed from the forwarded initial test-run arguments after being read into the mutation configuration.
Mutation child processes now distinguish assertion failures from execution errors. Exit code 1 is still reported as tested, while other non-successful exits are reported as errored.
The new errored result is wired through the result enum, counters, events, and printer output.

Testing

• composer test