Add pinned polymer diffraction peak inference

[NJMarchese]

Summary

• add BraggPeaksPolymer, peak detection, angular-origin/polar transforms, and calibrated flowline/figure helpers
• retain the checkpoint-exact paper architecture separately from the newer core CNN
• add checksum-verified pinned/latest model resolution with atomic downloads, offline cache reuse, and private local-directory overrides
• document the full dev/polymer overlap reconciliation

The default paper record remains intentionally unavailable for network download until its immutable public Zenodo record exists. This PR should remain draft until that URL is pinned.

Verification

• pytest -p no:cacheprovider -q tests/diffraction tests/datastructures/test_dataset4dstem.py (32 passed)
• interactive GPU job 56237850: strict paper checkpoint load and finite (1, 2, 256, 256) inference passed
• downloader tests cover pinned/latest, cache hit, interruption, incompatibility, and checksum failure

Scope exclusions

Grain clustering, generator/training launchers, research notebooks, generated results, and the private Zenodo archive are not included.