feat: implement variant association analysis

[31puneet]

Closes #685

Adds variant_association() performs Fisher's Exact Test between a specific SNP and resistance phenotype. Uses phenotypes_with_snps() internally, filters missing data, and returns odds ratio + p-value.

How it works

1. Fetches aligned phenotype + genotype data via phenotypes_with_snps() for a single coordinate (not the whole chromosome)
2. Filters by insecticide/dose/phenotype if specified
3. Removes samples with missing phenotypes (NaN) or missing genotype calls (-1)
4. Builds a 2x2 contingency table (has_alt/has_ref × alive/dead)
5. Runs scipy.stats.fisher_exact() and returns odds ratio + p-value

[31puneet]

Hi @jonbrenas PTAL, meanwhile when you get time see #1051