feat: Add mask-aware dataset grids for instance segmentation label validation

[pirnerjonas]

Summary

• render instance masks in saved dataset grids when targets include masks
• pass masks through supervision.Detections and draw them before boxes and labels
• add a regression test that verifies segmentation masks are forwarded to the mask annotator
• use Matplotlib's non-interactive Agg backend for dataset grid file rendering so CI and headless environments do not require Tk

Refs #1013

Validation

• uv run --no-sync pytest tests/datasets/test_save_grids.py -q
• MPLBACKEND=TkAgg uv run --no-sync pytest tests/datasets/test_save_grids.py::test_save_grid_passes_masks_to_supervision -q
• uvx pre-commit run --all-files
• GitHub PR checks passed on f5e5679, including Testing (windows-latest, 3.10)

[codecov]

Codecov Report

❌ Patch coverage is 72.97297% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 77%. Comparing base (7f2490d) to head (f5e5679).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #1014    +/-   ##
========================================
- Coverage       80%     77%    -3%     
========================================
  Files          100     101     +1     
  Lines         8447    8549   +102     
========================================
- Hits          6774    6569   -205     
- Misses        1673    1980   +307     

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