Fix AHO phase parity for numeric-only GT handling

[Jaureguy760]

Summary

• restrict analyze_imbalance GT phase parsing to numeric VCF-style phased hets (0|1 and 1|0)
• avoid deriving haplotype phase from allele-string GT values like A|G
• fall back to the unphased model when requested phased mode lacks informative numeric phase

Verification

• cargo fmt --check
• git diff --check -- src/lib.rs src/analysis.rs
• cargo test --lib: 116 passed; 0 failed; 7 ignored

Benchmark context

This is the Rust candidate commit for the fresh AHO-vs-Rust phase parity lock. AHO remains the ground truth; donor dispersion remains a Rust-only extension and is not treated as AHO parity.