feat: add compute_haplotype_statistics tool

[ameynert]

Summary

• Ports compute_haplotype_statistics.py from human-diversity-reference/scripts as a defopt-compatible toolkit tool
• Evaluates unique haplotype counts and gnomAD variant counts across all combinations of provided frequency cutoffs and window sizes
• Writes results to two TSV files (haplotype counts and variant counts)
• window_sizes and frequency_cutoffs are now list parameters (--window-sizes 100 500 1000) rather than comma-delimited strings

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_08_compute_haplotype_statistics

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