Add a worked recounted-rung example

[eduardstan]

Summary

• add a frozen synthetic reason-trace example reaching the recounted rung
• demonstrate that its self-reported reasons cannot be promoted to probed
• document the rung and pin the worked transcript and packaged module

Checks

• pytest --cov=reasonsmith --cov-fail-under=95 (1633 passed, 36 skipped)
• ruff check .
• mypy