Beta promotion. Headline change: panel-agnostic by structure rather
than by accident — HumanOrigins, chrY-only haplogroup, chrM-only
mtDNA, and arbitrary custom EIGENSTRAT panels all work end-to-end
without crashing on the 1240k-specific autosomal coverage gate.

Plus the LLD #19 full-chain f2 equivalence test (closes "verified
equivalent to mergeit's pipeline" claim) + validate-time flag-probe
checks (catches future tool-CLI drift before crashing 22s into Stage 3)
+ chr20-as-anchor build-detection fallback (chrY-only haplogroup BAMs).

249 tests passing; 6-cell + bio-tools CI matrix green. Item #4
(region-parallel mpileup fan-out) deferred to v0.3.

See CHANGELOG.md for the full history.