Log every scan run in drift log

[bradthx]

Summary

1. Add a scan completion logger entry for every scan, including clean scans.
2. Include scan timestamp, counts, and exit code in the log message.
3. Add regression test verifying per scan timestamped log entries.

Validation

1. python3 -m py_compile ndrift tests/test_ndrift.py
2. python3 -m unittest discover -s tests -p 'test_*.py' -q