Add Mindmap Brain segmentation model

[usmann56]

• What: port the mindmap model to the MONAI bundle format
• Why: MONAI-ecosystem interoperability (MONAI Label, MONAI Deploy, monai.bundle tooling), not a replacement for brainchopC's own engine
• Model background: 18-class whole-brain segmentation (gray/white matter and subcortical structures), MeshNet-style dilated 3D CNN -- model16chan18cls's topology at 24 channels, from the brainchopC / brainchop-models project (neuroneural lab)
• Planned validation approach: checked against brainchopC's own CPU-backend reference outputs (voxel-level agreement; not expected to be fully bit-exact, since an 18-class segmentation has far more inter-class decision boundaries than a binary mask)
• A note that the bundle is already implemented and a PR will follow