fix: remove animation spans after streaming ends

[BetterAndBetterII]

Description

Fix animation spans (data-sd-animate) persisting in the DOM after isAnimating goes false. Memoized leaf components (MemoParagraph, MemoLi, etc.) compare only className + source position and never compare children, so they discard the span-free reparse that Block produces when the animate rehype plugin is removed from the pipeline.

Root cause

Block comparator correctly detects the rehype pipeline change and re-parses without the animate plugin. But the resulting span-free HAST never commits because leaf memo comparators (sameClassAndNode) see identical class + position and return "equal" — React throws away the new children.

Fix

Include isAnimating in block keys. When streaming ends, keys change and React mounts fresh subtrees, allowing the span-free reparse to commit. During streaming (isAnimating stays true), keys are stable — no hot-path penalty.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Related Issues

Fixes #570

Changes Made

• packages/streamdown/index.tsx: Include isAnimating in block key computation so React remounts fresh subtrees when the animate rehype plugin is removed
• packages/streamdown/__tests__/animate-integration.test.tsx: New integration test verifying spans are removed when isAnimating goes false
• .changeset/clean-settled-animation-spans.md: Patch changeset

Testing

• All existing tests pass (983/983, 74 files)
• Added new integration test: animate-integration.test.tsx
• RED confirmed: test fails without fix (19 spans persist)
• GREEN confirmed: test passes with fix (0 spans)
• No regressions in full test suite

Checklist

• My code follows the project code style
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes
• I have created a changeset for these changes

Additional Notes

The fix only affects the transition when isAnimating flips. During active streaming (isAnimating stays true), block keys remain stable — no extra remounting on the hot path. The one-time remount cost when streaming ends is negligible compared to the persistent DOM pollution of stale animation spans.

[vercel]

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[vercel]

Additional Suggestion:

blockKeys unconditionally includes isAnimating in every block key, forcing a full unmount/remount of all blocks when streaming ends — even when the animated prop is absent and there are no animation spans to discard.