[NO-ISSUE] feat(webkit): premium linear shimmer for Skeleton

[gab-az]

Summary

Replace the Skeleton placeholder's opacity pulse with a premium linear shimmer — a gradient highlight band that sweeps diagonally across the fill.

• theme: add a shimmer preset to the animate token object (--animate-shimmer: shimmer 1.6s linear infinite) and the matching @keyframes shimmer (background-position sweep) to semantic/animations.js. Rebuilt dist/v3 so the variable lands in globals.css/globals.scss.
• webkit: Skeleton now renders a linear-gradient(45deg, ...) band between --bg-surface-raised and --bg-surface, sized 200% 100% and driven by animate-[var(--animate-shimmer)]. The whole effect is gated behind data-[animated]:motion-safe: (flat fill under reduced motion) plus an explicit motion-reduce:animate-none.
• Spec .specs/skeleton.md updated to match (Motion table, Theme gaps, JSDoc) with a recomputed checksum.

Notes

• No new dependencies. CSS + tokens only.
• No public API change (no prop/event/slot/export added or removed) — bump is minor from the new feat commits.
• --animate-shimmer still has a TODO to be documented in DESIGN.md § Animations