For agents, on top of README.md, which they read first.

• A font file is replaced only by another subset of the same face with the same name; the stylesheet's @font-face and asset::FONTS name the paths, and a test holds them together.
• The stylesheet may not contain a :// address or an @import (tested).
• datastar.js is never edited here.
• pixel-emoji.woff2 holds exactly the codepoints of components::PixelEmoji and of the unicode-range in ui.css; add one in build-pixel-emoji.py, the enum and the range together (a test holds the last two). Its licence file goes with it.