feat: skeleton — dithered loading placeholders (chart-silhouette variant)

[DloomPlz]

Summary

Adds <DitherSkeleton>, a loading placeholder in the kit's own texture: a slow shimmer band sweeping through the ordered dither instead of a flat pulsing rect.

• variant="block" — size it like any skeleton (className="h-4 w-40 rounded"), text lines / cards / avatar squares.
• variant="chart" — draws a seeded area silhouette (two seeded sine waves via the kit's fnv1a/xorshift32), so a chart slot loads "in character". Same seed → same shape, so a reloading chart never pops between two different placeholder silhouettes.
• Standalone registry item like avatar/button/gradient — shares only pixel.ts + palette.ts, no chart engine.
• Honors prefers-reduced-motion (static frame, no loop), always aria-hidden (placeholders are decorative), cleans up rAF + ResizeObserver, backing resolution capped.

Details

• Pure models (skeletonHeights, shimmerLift) are exported and unit-tested; a mount test pins the aria-hidden contract (happy-dom safe — the painter no-ops on a null 2D context).
• Registry item + barrel export + COMPONENTS.md install/usage sections; r/ JSON regenerated with npm run build:registry.
• Suite: 30/30.

Battle-tested in a production dashboard where it replaced every animate-pulse chart placeholder.

[tripwire-dev]

passed — nothing tripped. good to merge.