WarpTransform: reflection padding to remove edge striping at warped-axis faces

[pscamillo]

Follow-up to the scroll-specific Warp augmentation (merged in #999, issue #201).

While validating WarpTransform on a real Scroll 5 CT crop, I found a thin striped band along the warped-axis end-faces. Root cause: the coherent displacement is constant along the normal axis (by design — the stacked sheets bend together as a unit), so at the two end-faces the shift pushes the sampling grid off-volume. With padding_mode="border", those out-of-bounds samples clamp to the single edge value; since the displacement varies in-plane, adjacent columns clamp by different amounts, leaving a striped band at the face.

Fix: switch the grid_sample in _warp from padding_mode="border" to "reflection". Out-of-bounds samples now mirror interior texture instead of replicating the edge value, so the band disappears. The displacement field is untouched, so the coherent (constant-along-normal) bend and the strictly-monotonic / no-fold guarantee are preserved. One-line change.

Validation on a real Scroll 5 crop (1, 48, 512, 512), with the normal axis forced to an in-plane axis so the face is visible in the viewed plane:

• before (border): striped band at the warped-axis face
• after (reflection): face is clean; interior warp unchanged

The __main__ self-test still passes (identity when disabled, shape preserved, finite values, labels a subset, coherent bend).

[vercel]

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