Fix dark-mode augmentation and simplify augment-vis
- _bg_color: sample image corners to detect background color
- _find_blocks: adaptive ink threshold for dark/light mode
- _transform_patch, _region_jitter, _augment: use bg color for all
canvas fills instead of hardcoded white; elastic transforms now use
pad-apply-crop to avoid white-fill artifacts
- augment_vis: simplify to single _augment call matching training exactly
- Update DVC manifests for regenerated typeset splits
- Expand _EMOJI pool with directional arrows, pin, food items
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>