feat(registry): mk-background — Apple-keynote procedural gradient background

[jbernard077]

What

A new registry block: mk-background — an Apple-keynote-style procedural gradient backdrop.

• Two soft radial blobs (position, radius, color, opacity via CONFIG) drifting slowly over a base color — the gradient is fully procedural, no baked assets
• Bar mask: an animatable rounded-rect card — the default 10s timeline showcases full-bleed → floating card over the stage → full-bleed
• Light and dark schemes (#f5f5f7 stage / white base vs. black / violet blobs), all colors exposed as --mk-* CSS custom properties so a host can re-skin without editing the block
• Optional frost mode (contentOpacity + frostBlur) for the frosted-card look over footage
• Seek-safe: single paused GSAP timeline, no rAF, no randomness

Why

First of a planned family of minimal "keynote-style" presets (backgrounds, pill callouts, spec lists, karaoke-highlight typography, placeholder grids, a text-clone-wall transition). This block is the foundation the rest demo on. The design language is a clean-room re-implementation of the aesthetic conventions popularized by Apple keynotes (and preset packs inspired by them) — procedural soft-blob gradients, Inter typography, restrained motion. No third-party assets or code were copied.

Validation

• hyperframes lint → 0 errors, 0 warnings
• hyperframes validate --no-contrast → no console errors
• Layout inspector → 0 issues (blob overflow of the clipping card is intentional and marked data-layout-allow-overflow)
• Rendered headless to MP4 (10s) — preview attached below

Notes for maintainers

• External contribution — per CONTRIBUTING, the catalog PNG/MP4 upload needs a maintainer; preview MP4 is attached in a comment below
• docs/catalog/blocks/mk-background.mdx was generated with scripts/generate-catalog-pages.ts. I did not commit the wider regeneration diff (docs.json / catalog-index.json), since the committed generated files appear to lag the generator and I didn't want to sweep unrelated churn into this PR

🤖 Generated with Claude Code

[jbernard077]

Preview render (10s):

preview-mk-demo.mp4