Add SlicerPETTumorSegmentation extension

[gefeichen]

Summary

• Add catalog entry for SlicerPETTumorSegmentation, a scripted extension for quantitative PET/SPECT tumor segmentation.
• Source: https://github.com/gefeichen/SlicerPETTumorSegmentation
• Tier 1 (Experimental). Includes DICOM support rule for PT/NM modalities.

Tier 1 checklist

• Extension has a reasonable name: SlicerPETTumorSegmentation
• Repository name follows Slicer+ExtensionName convention
• Repository topic 3d-slicer-extension added
• Extension description summarizes purpose in README
• No known related patents
• LICENSE.txt (Apache-2.0) in repository root; license named in README/homepage
• scmurl/scmrevision correct (master branch)
• Extension icon URL points to raw GitHub PNG (128x128)
• Screenshot URL included
• JSON content consistent with top-level CMakeLists.txt
• Homepage (GitHub README) contains name, description, screenshot, module list
• Extension is safe: no unapproved binaries; pydicom installed only on user opt-in

Modules

• SUV Converter — DICOM metadata audit and SUVbw conversion
• SUV Tumor Segmentation — ROI-based absolute/relative threshold segmentation

Test plan

• Dashboard builds extension on Windows, macOS, Linux
• Install from Extension Manager on Slicer 5.10
• SUV Converter inspects a PT DICOM series
• SUV Tumor Segmentation places ROI and previews threshold segmentation

[gefeichen]

Withdrawing for local review before resubmission.