feat: animation effects and @streamdown/effects (diffuse, scramble)

[thedonmon]

Description

Lets animated.animation take an effect object, and adds an @streamdown/effects package with two effects:

• diffuse: words sharpen out of a blur, in scattered order.
• scramble: words start as noise glyphs and resolve to the real text, in scattered order.

import { scramble } from "@streamdown/effects";
import "@streamdown/effects/styles.css";

<Streamdown smooth animated={{ animation: scramble }} isAnimating={isStreaming}>
  {markdown}
</Streamdown>

Type of Change

• New feature (non-breaking change which adds functionality)

Related Issues

Depends on #619 and #625. This branch includes their commits, so it stays a draft until they merge. Without #619, a re-render resets words that are still animating, and smooth re-renders every few frames, so the effects never show.

Changes Made

• Core: AnimationEffect is { name, duration?, scatter?, decorate? }. scatter adds a stable per-word delay for a scattered order. decorate adds data-* attributes while a word animates; other attribute names are dropped. String animation names work as before.
• New @streamdown/effects package with no dependency on streamdown.
• scramble keeps the real text in the DOM, so copy and screen readers get real words. It falls back to a fade with prefers-reduced-motion.
• Docs and changesets.

Testing

Screendrop_2026-09-19-09-40-08_27B738.mp4

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Checked both effects in Chrome with smooth on headings, lists, tables, inline code, strikethrough and math.

Additional Notes

This adds a new published package. If you'd rather not, the two effects could live in core instead.

🤖 Generated with Claude Code

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