Implement multigwas for multiallelic GWAS

[alex-sandercock]

Enable mixed-model GWAS for multiallelic loci (3+ alleles per marker).

Tasks:

• Extend genotype layer to support multiallelic sites
• Define MarkerEncoder trait for multiallelic encoders
• Implement multiallelic encoding strategies (reference-based dosage, K-1 columns for K alleles)
• Implement run_multigwas() using shared mixed-model engine