Add --count/-c flag to 'uwf thread step' for running N steps in one
invocation, stopping early if $END is reached.
- cmdThreadStep now loops up to count times, delegates to cmdThreadStepOnce
- CLI parses -c/--count, defaults to 1 (backward compatible single output)
- Validation rejects 0, negative, and non-integer counts
- 7 new tests covering CLI parsing and count validation
Fixes#373
Co-authored-by: uwf-hermes (solve-issue workflow)
xiaoju