feat: add wide variant-TSV header and per-pop validation

[ameynert]

What

Append wide variant-TSV validation to create_duckdb_from_tsv.py: a header check
(validate_variants_header) and a legend-driven, null-safe per-population validator
(read_and_validate_variants), plus a variants.tsv fixture and a parametrized refusal-table test.

Why

The pure-Python TSV path must reject malformed input up front. Validation is vectorized (polars) and
null-safe:

• Header: the fixed contig,pos,ref,alt plus AC_<pop>/AF_<pop> per population must be present;
  an AC_/AF_ column naming an unknown population is rejected; other extra columns (e.g. rsid)
  are permitted and ignored.
• Rows: pos >= 1; contig a GRCh38 main-contig token; ref/alt are A/C/G/T/N; each pop's
  AC_/AF_ is both-present-or-both-empty with AC >= 0 and AF in [0, 1]; at least one defined
  AF per row. Failures name the first offending (contig,pos,ref,alt); a null-derived comparison
  counts as a failure (fill_null(False)), never a silent pass.

Notes

• Fixture variants.tsv covers a SNP, a missing-population cell, an insertion, and a deletion.
• Stacked on feat: add SourceMetadata sidecar model and loader #127. Base: feat/tsv-source-metadata. Draft.

Related to #114