Design rain that arrives on a lap the seed picks #332

Part of #322. A design issue: no build until the mockup and ADR are agreed.

Problem

Nothing changes during a race; Mario Kart World and F1's best races turn on a change mid-race.

Goal

Decide whether and how rain arrives on a lap the seed picks: what grip, tyre life and the pit lane do in the wet, how the sky, road and spray read in an eight-kart pack, and what bots do.

Change

• A mockup under docs/mockups/ comparing two or three versions (grip only; grip and a wet tyre choice in the pit lane; visual rain only).
• An ADR for the chosen version, then build issues.

Done when

A version is chosen and recorded, or rain is dropped with the reason recorded.

Spec

docs/mockups/course-excitement.html on branch docs/course-excitement-picks until its PR merges: card X6, with pi's objection on it.