meshseg v2 (#410): surface-sampled synthetic + CC0 mined rigs, 94.7% vs 31.5% on rig-truth eval

Restore original segmentation model (revert in-progress rework that didn't improve quality)

meshseg v1.2.0: flat-PointNet (runs correctly in C++ ORT) + CC0 real-data training

Improve mesh-seg model: T/A-pose data + upright aug + PointNet++ local features (#410)

Add mesh part-segmentation model (#410) — synthetic humanoid (CC0)