Complete Role D: factorial support, import fix, git recovery tasks, and history cleanup

[tngwafor]

Summary

This PR completes all Role D requirements and describes my contributions.

Code changes

• Fixed the missing import in tests/test_calculator.py
• Updated .gitignore to ignore Python cache files
• Reverted the bad debug logging commit without removing it from history
• Added the is_positive helper function to src/validator.py
• Completed the reflog recovery exercise and merged recovered work

Git workflow tasks completed

• Created feature branch from starter/role-D
• Used separate commits for unrelated CP1 changes
• Reverted the BAD: commit with git revert
• Moved a commit from main to the feature branch with git cherry-pick
• Recovered deleted branch work using git reflog
• Rebased feature branch onto updated main
• Cleaned commit history with interactive rebase

Validation

• pytest tests/ passes locally

Notes

• LEARNINGS-tngwafor.md is included in this branch
• Commit history was cleaned up before submission

Checklist

• Checkpoint 1 complete
• Checkpoint 2 complete
• Checkpoint 3 complete
• Checkpoint 4 complete
• Checkpoint 5 complete
• Checkpoint 6 complete
• Tests passing

[ngangi0195]

The PR description is thorough, but it would help to add a short 1-2 sentence summary at the top explaining what this branch is and why it's being merged, rather than just listing what was done.