fix(mobile): smooth Android scrolling through long settled threads

[macodev00]

What Changed

On Android, a long settled assistant message is no longer one list row. It is split into bounded markdown rows, and a code fence longer than 16 lines is windowed into plain text rows. Android also recycles transcript rows. iOS is unchanged.

Each markdown slice is parsed as its own document, so slices are cut only where that stays valid:

• Lists, blockquotes, GFM tables, HTML blocks, and setext headings stay intact, including lazy continuation.
• Inline links, images, emphasis, inline code, autolinks, and raw HTML are not cut. Nested elements of the same name stay balanced.
• Link reference definitions are copied onto a slice that uses them, including a destination or title on the following line.
• Indented fences drop the opening fence's indent from each content line. An empty fence still renders.
• <textarea> blocks stay intact through </textarea>.
• A backtick fence whose info string contains a backtick stays prose.
• A construct longer than the budget stays one row instead of being hard-split.
• Short messages, and a single short fence, stay on the highlighted renderer.
• Recycled code rows key the horizontal scroller and copy button by the full fence text, so a shared language and opening characters cannot keep the previous scroll offset or copied state. Streaming appends keep that key stable.

Fixes pingdotgg#13925

Why

Scrolling a long settled thread on Android stalls on the UI thread while Fabric mounts one large selectable text tree, then mounts it again when the row comes back. Smaller rows plus container reuse remove that hitch. Slice boundaries stay on markdown constructs so a link, list, quote, or code fence is not rendered as plain prose.

UI Changes

Android transcript scrolling only. This environment has no Android emulator, so there are no before/after frames. Slice boundaries are covered by apps/mobile/src/features/threads/androidTranscriptSlices.test.ts.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

  

[macodev00]

Opened upstream.