#21371 fix(agent-pane): settle follow at the streaming bottom

Summary

Narrow the WebKit follow repair to the current feat-cmux-next base. Streaming bottom-following now uses a 2 px resting deadband and only writes a meaningful correction, preserving the follow trace across a second composer send without carrying the unrelated sidebar/motion changes from #21331.

Validation

• bun test test/transcript-follow.test.ts (browser lane skipped outside CI/Freestyle)
• CMUX_BROWSER_TESTS=1 bun test test/transcript-follow.test.ts (local DOM timeout while starting the Vite browser lane; hosted WebKit remains required)
• bun run typecheck (no type errors in 1482 files)
• git diff --check

The existing transcript-follow browser test already asserts the <=2px follow tolerance. Hosted WebKit/Chromium validation is required on this exact head before merge.

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Summary by cubic

Sets bottom-following to a 2px resting deadband in the agent transcript, so streaming replies no longer write fractional scroll offsets and twitch the thumb.

• The new settleFollowBottom helper only corrects when displacement exceeds the deadband, which also preserves the follow trace across a second composer send.
• Applies only the follow repair onto the current feat-cmux-next base; unrelated sidebar and motion changes are not carried over.