Add BraTS Mets panoptica tutorial with Standard PQ and Part PQ

[aymuos15]

Summary

• Adds a new Panoptica tutorial for BraTS Metastasis 2025 segmentation evaluation
• Demonstrates per-sub-region Standard PQ (Necrotic, Edema, Enhancing evaluated independently)
• Demonstrates Part PQ with Enhancing Tumor (3) as the thing and Necrotic Core (1) as the part, with Edema (2) evaluated as a standard LabelGroup
• Includes threshold sweep comparing Standard PQ vs Part PQ across IoU thresholds 0.05–0.95