Genotype population priors: Genotype.sample(...)

[MuteJester]

Summary

Follow-on #1 to the merged genotype foundation. Adds Genotype.sample(cfg, …) — draw a fully-specified diploid genotype from population priors instead of specifying every gene by hand.

• Model: independent per-gene, per-chromosome Hardy-Weinberg draw — each gene on each chromosome is independently deleted (haplotype_deletion_prob) or assigned an allele from that gene's frequencies; homozygous/heterozygous/hemizygous/deleted emerge at the expected rates. Deterministic (seed).
• Honest scope: NOT a population haplotype model (no LD, co-deletion blocks, ancestry, donor haplotype structure); catalogue alleles only (no novel); deletion only (no duplication). Documented as such.
• Frequencies: uniform-within-gene default (deliberately not the cartridge's recombination allele_usage); opt-in allele_frequencies="usage_as_prior"; segment-aware {segment:{gene:{allele:weight}}} (+ flat {gene:…} when unambiguous); zero weight excludes an allele; full name/weight validation.
• Deletion: float or per-gene/per-segment dict, validated to [0,1].
• Always runnable: segments_to_sample must cover required segments (no half-specified genotypes); ensure_viable=True (default) re-draws deterministically up to max_resamples until a complete haplotype exists, else a clear ValueError; ensure_viable=False opt-out.
• Docs: new "Sampling from population priors" guide section (with honesty note); removed from Limitations. All 11 doc examples execute against the live API.

Test plan

• CI: full Rust + Python matrix green
• 55 genotype tests (determinism, HW-rate bias, zero-weight exclusion, deletion shapes, viability, truth-call end-to-end). Full local Python suite: 2369 passed. mkdocs --strict clean. No engine/Rust changes.