fix: pin drift snapshot checkout revision

[Pigbibi]

Summary

• add an optional snapshot_repository_ref reusable-workflow input
• checkout the exact artifact producer revision when callers provide it
• retain main as the backward-compatible default

Verification

• PYTHONPATH=src python3 -m pytest -q tests/test_reusable_drift_workflow.py
• actionlint .github/workflows/reusable-drift-check.yml
• git diff --check

[github-actions]

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