Fix hatch-pet chroma-key halo and webp transparent-RGB loss

[shmulc8]

Summary

Two related fixes in the curated hatch-pet skill that together cause a magenta (or other chroma-key) tinted halo around extracted pet sprites in the final spritesheet.

Root cause

1. scripts/extract_strip_frames.py — remove_chroma_background only zeros alpha on chroma-keyed pixels and keeps the original RGB:

   pixels[x, y] = (red, green, blue, 0)

   The subsequent fit_to_cell resize uses Image.Resampling.LANCZOS, which interpolates RGB and alpha independently. Transparent pixels still carrying chroma RGB then leak color into neighboring partially-opaque edge pixels during downscaling, producing a faint tinted halo (e.g. (127, 0, 127, 2), (85, 0, 85, 3) for a magenta key) around every sprite.

2. scripts/compose_atlas.py — save_outputs writes the WebP atlas with lossless=True but without exact=True. Without exact=True, libwebp is free to overwrite RGB on fully-transparent pixels during compression for better packing. Combined with Update readme with initial text; add gitignore #1, this can also re-introduce non-zero RGB on alpha=0 pixels in the published atlas, which any renderer that ignores the alpha channel will display as solid chroma-key color.

Fixes

1. In remove_chroma_background, set chroma-keyed pixels to (0, 0, 0, 0) so the resize cannot pull chroma color into sprite edges.
2. In save_outputs, pass exact=True alongside lossless=True so transparent-pixel RGB is preserved as-written.

Both changes are tiny and surgical — no behavior change for non-chroma-key pixels.

Test plan

• Run extract_strip_frames.py on a strip with a magenta (#FF00FF) chroma-key background and verify post-extraction frames have no (R≈B, G≈0, alpha>0) pixels along sprite edges.
• Compose an atlas via compose_atlas.py --webp-output ... and verify with PIL that there are no chroma-family RGB values on alpha==0 pixels in the saved WebP.
• Visually check the final spritesheet in a renderer that ignores alpha (e.g. some sprite preview tools) — there should be no solid chroma-key fill where transparency is expected.