Add CapHLA presentation and affinity wrapper

[iskandr]

Summary

• add CapHLA, CapHLA_EL, and CapHLA_BA for class-I/class-II EL presentation and BA affinity
• fetch the exact MIT-licensed upstream commit and five-fold weights through mhctools fetch caphla; report it through mhctools ls
• preserve both native outputs in the canonical API, including BA normalized score plus inverse-transformed IC50 nM, with no invented ranks or thresholds
• normalize and validate all 16,025 upstream HLA/H-2 library identifiers through mhcgnomes-derived identities
• load unchanged upstream model classes/checkpoints lazily, batch in-process with deterministic zero-worker behavior, and support EL-only, BA-only, or both
• add CLI registry entries, a caphla optional extra, user documentation, and a dedicated public integration job

Verification

• ./lint.sh
• ./test.sh — 718 passed, 55 skipped, 2 xfailed
• managed mhctools fetch caphla smoke test — 44 MB installed with pinned provenance
• official 32-row EL+BA regression — maximum tolerated absolute error 3e-7
• all upstream alleles resolve to a present library key: 10,389 class I + 5,636 class II
• real class-I, class-II heterodimer, and H-2 predictions through the managed snapshot

Closes #270.

Related parser API gap: pirl-unc/mhcgnomes#190.