Community projects: add Ring + Streak CT-artifact augmentations (#201)

[pscamillo]

Updated the description of scroll-specific augmentations to include additional features and validation details.

[vercel]

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[pscamillo]

Both transforms referenced here are now validated on a real raw CT crop from Scroll 5 (PHerc172, open zarr store), not just the synthetic self-tests — before/after plus per-voxel difference across depth.

• Ring (Add RingArtifactTransform: GPU CT ring-artifact augmentation (#201) #1020): concentric rings in the reconstruction plane, additive over papyrus; floored radius_range to (0.1, 0.9) so a ring never degenerates into a central blob.
• Streak (Add StreakArtifactTransform: GPU CT streak-artifact augmentation (#201) #1021): straight parallel directional streaks, additive over papyrus, no edge artifacts.

For both, when wiring into the scroll trainer I'd constrain ring_axes / streak_axes to the acquisition scan axis so the artifacts lie in the true reconstruction plane (the None default randomizes across all spatial axes). GPU cost on RTX 5070 (Blackwell sm_120): Ring 0.379 ms/call, Streak 0.401 ms/call for a (1, 192, 192, 64) patch. Per-transform detail and montages in the individual PRs.