feat: add smooth prop to pace bursty streams

[thedonmon]

Description

Some providers and relays send text in bursts, for example ~150 characters every ~550 ms. Rendered as it arrives, the text jumps and then pauses, and the caret jumps with it. smooth reveals each chunk a word at a time, at the rate chunks have been arriving.

<Streamdown smooth isAnimating={isStreaming}>
  {markdown}
</Streamdown>

Type of Change

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

Changes Made

• New smooth prop, off by default. Only appended text is paced; any other change renders immediately.
• A word split across two chunks is held until it's complete.
• When isAnimating goes false, the rest appears within 250 ms. Streamdown stays in streaming mode until then, even if mode switches to "static".
• With animated, stagger defaults to 0 under smooth, since pacing already spaces the words. An explicit stagger still applies.
• Docs page and changeset.

Testing

Screendrop_2026-09-19-09-37-30_CB5B1E.mp4

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

Replayed a captured stream (37 chunks, ~550 ms apart). The text now appears in ~700 steps of ~7 characters instead of 37 jumps of ~148. The cost is about one chunk interval of delay: median 583 ms before a chunk is fully shown.

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

🤖 Generated with Claude Code

[vercel]

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[vasilich6107]

Hi @farnabaz @haydenbleasel 👋 — a friendly +1 for this one.

We're hitting exactly the case in the description: our chat relays an agent's answer in ~500 ms chunks, so on 2.6.0 it renders as a jump followed by a pause.