Add predictor MHC context metadata

[iskandr]

Summary

• Add predictor-level kind_support() metadata with direct per-kind mhc_dependence and mhc_class fields.
• Document MHC context semantics for single-allele, haplotype-level, and MHC-independent predictions.
• Change the MHCflurry presentation wrapper to call the upstream presentation predictor once with the full requested allele set and emit one haplotype-level presentation prediction per peptide, using best_allele as attribution.
• Add regression coverage for MHCflurry haplotype presentation behavior, predictor metadata, and processing predictors being MHC-independent.

Root Cause

MHCflurry’s presentation predictor is genotype/haplotype-aware, but the mhctools wrapper was calling it one allele at a time and exposing those outputs as if presentation were per-allele. That changed the wrapper contract relative to the upstream model and made Kind.pMHC_presentation semantics ambiguous for downstream tools.

Validation

• ./lint.sh
• ./test.sh (414 passed, 9 skipped, 2 xfailed)

Fixes #208
Fixes #209