Integrating brachial plexus & DRG segmentation

Propose integrating a new model for automated bilateral segmentation of the brachial plexus and dorsal root ganglia at cervical levels C5–C8 from 3D STIR sequences

Dataset: approx. 400 total participants (340 on Siemens, 60 on GE), including 3 total sites

Model details:
- Architecture: Cascaded nnU-Net (2-stage, 5-fold ensemble)
- Stage 1: 3-class coarse segmentation (background / DRG / plexus)
- Stage 2: Fine 16-class bilateral segmentation (BP C5–C8 + DRG C5–C8, bilaterally [separate left & right])

Start by surveying the repository's existing model integration entry points and conventions, since the issue names no files or tests. Compare the proposed cascaded nnU-Net stages, label scheme, imaging data, and multi-site setup with current project support. Done means the new bilateral C5–C8 plexus and DRG model is integrated and its behavior is validated according to the repository's existing checks.

Written by the indexing model from the issue text.