Fetch pinned predictor tools and weights

[iskandr]

Summary

• add atomic, commit-pinned sparse fetches for DeepTAP, DeepImmuno, ERAMER, TULIP-TCR, NetTCR, and BigMHC
• require explicit license acceptance for NetTCR and BigMHC, and preserve repository, revision, sparse-path, and license provenance in each installation
• teach wrapper constructors and class-level fetch() methods to use the managed snapshots automatically
• expand mhctools ls to inventory bundled weights, MHCflurry-managed downloads, mhctools-managed snapshots, user checkouts, and manual licensed executables
• add MHCTOOLS_DATA_DIR / --data-dir, JSON-safe fetch output, and TULIP setup-script integration

MixMHCpred, MixMHC2pred, PRIME, DTU tools, NetCleave, and TLimmuno2 remain inventory-only because mhctools cannot safely redistribute them under their current terms.

Verification

• ./lint.sh
• ./test.sh — 699 passed, 53 skipped, 2 xfailed
• real ERAMER fetch and wrapper resolution
• real NetTCR sparse fetch: 20 checkpoints, 7.9 MB installed, all checkpoints load
• NetTCR upstream-reference regression: 28 passed, including all six published ensemble anchors

This builds on #271 and removes the artifact-acquisition part of iskandr/eureka-bench#159.