Add start_process(name) following supervisor's start rules: STOPPED and
BACKOFF clear manual_stop, drop any pending retry, and spawn now; RUNNING and
STARTING report success without acting; STOPPING is rejected so the caller
retries. Returns (ok, message).
Add tests for the stopped, backoff, running, stopping, and unknown cases.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>