feat: Add coordinate-aware CNN layer 0 for position-dependent stylization

- Implement CoordConv2d custom layer accepting (x,y) patch center
- Split layer 0 weights: rgba_weights (9x mat4x4) + coord_weights (mat2x4)
- Add *_with_coord() functions to 3x3/5x5/7x7 convolution shaders
- Update training script to generate coordinate grid and export split weights
- Regenerate placeholder weights with new format

Size impact: +32B coord weights + ~100B shader code = +132B total
All 36 tests passing (100%)

handoff(Claude): CNN coordinate awareness implemented, ready for training

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 0 insertions, 0 deletions

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