The 3D board textures can be extracted as WebP: each player downloads about 4.7 MB less on a cold start (#186)

Only the extraction (tools/local-extract) and the manifest pointer change; no official textures are committed and nothing in play changes. Contributed by @Cloudnyco.

docs/ASSETS.md

@@ -129,7 +129,7 @@ The `stem` of a Spine model is the upstream file name. Two examples: `char_107_l
 - **Fonts:** OTF/TTF files are converted to WOFF2 by a built-in encoder (`tools/assets/woff2.mjs`: Brotli with null transforms).
   - Its output was verified lossless against Google's reference `woff2` decoder.
   - `fonts.css` lists WOFF2 first and falls back to the original file.
-- Images stay PNG. WebP conversion is not done: it would need a native dependency.
+- Images stay PNG. WebP conversion is not done: it would need a native dependency. The local-client board textures are the exception: `tools/local-extract/extract.py` (Python, where Pillow is already a dependency) writes WebP copies of the 12 textures the 3D board downloads (`WEBP`: colour maps lossy at quality 95 with the alpha and the RGB under transparent texels kept, normal and data maps lossless) and `data/local-assets.json` lists the copies; the PNGs stay beside them for `tools/crop-board-atlas.mjs`. `extract.py --webp` adds the copies to an existing extraction without the client.
 
 ## Manifest schema (`data/assets.json`)