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.
• Link reference definitions are copied onto a slice that uses them.
• 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.

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. The previous attempt cut text at arbitrary offsets, so a link, list, or quote that crossed a boundary rendered as plain prose. This split refuses those cuts.

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.