The habit for a new scene

When you add a new scene or preset:

1. Eyeball it first — --preset <name> --out /tmp/new.png and Read the PNG.
2. Add the differential cases — the reactivity, animation, and sanity tests iterate the embedded presets automatically, so a new preset is covered once it’s in the default set; a new system may need its per-system floor in sanity. If it’s beat-driven, extend beat.
3. Check distinctness — run --report (or the distinctness test) to see if the new preset is a near-duplicate of an existing one (advisory).
4. A new system needs a golden fixture; a new preset does not. Adding a SystemKind variant fails golden.rs to compile until you author core/tests/fixtures/<system_name>.toml and add its arm — then bless that one baseline after eyeballing it. Shipped presets are never pixel-pinned (ADR-0023).