feat: add compute_variation_ratios tool

[ameynert]

Summary

• Ports compute_variation_ratios.py from human-diversity-reference/scripts as a defopt-compatible toolkit tool
• Counts per-sample non-reference variant sites above each of seven predefined gnomAD population frequency thresholds
• Writes a sample-level Hail table with population labels and counts
• Removes the debug pprint call present in the original

Test plan

• uv run --directory divref poe check-all passes

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch am_09_compute_variation_ratios

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