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

Notes for agents

• Motion::all() lists every variant for the stylesheet; a test fails if a scene uses one missing from the CSS. Add a variant to all(), name, period, at and (if it should rest elsewhere than the identity) rest.
• A group's origin is in grid units and is the pivot for both the SVG attribute and the CSS transform-origin; keep them in step by only setting it on the node.
• The eyes' gaze factors live in scene.rs (GAZE_REACH, GAZE_STEP) and the pointer expression is built from them; never type those numbers elsewhere.