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`)
 

docs/DEPLOY.md

@@ -257,3 +257,5 @@ services:
 表情（6 套 × 6 个）和「玩法说明」的 19 页教程图公开镜像也有：`node tools/setup.mjs` 会和其他素材一起下载（约 21 MB），不需要客户端；有本地素材时优先显示本地的。
 
 **没有客户端的服务器**想要上表中的官方素材：从**同一版本**的完整包（[Releases](https://github.com/sganggs/Stronghold-Protocol/releases)）里，把 `public/assets/local/` 文件夹和 `data/local-assets.json` 复制到服务器项目目录下的相同位置。服务器每次请求都会重新读取这两处，不必重启，玩家刷新页面即可。一定要用与服务器代码相同版本的完整包：各版本提取的内容和清单可能不同（例如灼热 / 炽焰源石虫的模型是 0.1.0 之后才加入的），混用其他版本的文件会缺图或用错图。复制后 `node tools/doctor.mjs` 会显示本地素材的条目数和「3D 棋盘可用」。
+
+**3D 棋盘贴图的下载量**：每位玩家进入对局时都要从开服的电脑下载 3D 棋盘的 12 张贴图。提取时会给这 12 张各写一份 WebP（颜色贴图有损、质量 95，法线和数据贴图无损），清单里列的是 WebP，同名 PNG 留在旁边给裁切工具和 setup 用。这部分下载量从约 6.7 MB 降到约 2 MB，网速慢的远程联机最明显。只有 PNG 的本地素材（例如在这一改动之前提取的）可以用提取时的 Python 环境运行 `tools/local-extract/extract.py --webp` 就地补上，只需要 Pillow，不需要客户端。