fix: marker, hr and img streaming animation

[shadcn]

Description

fix issue with list markers, task-list checkboxes, images, and hr animations

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Changes Made

• Fade list markers in via ::marker color, synced to the item's first word.
• Animate task-list checkboxes and <img>/<hr> via the standard data-sd-animate rule.

Testing

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

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

[vercel]

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Project	Deployment	Actions	Updated (UTC)
streamdown	Ready	Preview, Comment, Open in v0	Jun 22, 2026 4:11pm

[vercel]

Trailing void elements (img/hr) re-animate one extra time during streaming because they never advance charCounter.count, causing a boundary collision at count === prevContentLength.