`<Cascade>` was the last component using `animation` as its preset selector — a
vestige of the coordinated `cascade-*` system retired in Plan A. Every other
animatable component (Dialog, Popover, Drawer, …) already uses `motion`. Renames
`<Cascade>`'s prop + its context field `animation` → `motion`, so the framework
has ONE universal selector name (RFC §D.12 step a). `MotionPresetName` and the
low-level `data-animation-style` attr are unchanged.
- cascade: types `animation?` → `motion?`; context `CascadeContext.animation` →
`.motion`; component destructure + getter; item reads `ctx?.motion`.
- docs/demo: README props table + usage, index usage, panel-cascade demo.
Verified at runtime: `<Cascade motion="scale-fade">` produces identical wiring —
items carry `data-animation-style="scale-fade"`, `animation-name: scale-in,
fade-in`, structural index 0/1/2/3. `check` clean for the touched files; no
dangling `animation`-prop reference remains (only the `data-animation-style` attr).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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