## Why
The transcript overlay separately generated cell content for height measurement and rendering. Active cells backed by external state could also leave the live tail stale when their revision key did not change.
## What changed
- Share retained text layouts between measurement and painting, and render only visible rows while preserving wrapping, styles, and hyperlinks.
- Reuse stable layouts across frames, invalidating them when width, animation, syntax theme, or terminal colors change. Bound retention by entry count and text size, allowing a single oversized entry.
- Refresh externally mutable cells each frame and bypass live-tail reuse when any active source is not cacheable.
## Testing
Add coverage for layout reuse, eviction, invalidation, and live-tail refresh without a revision change. Expand the transcript snapshot to cover styled text, wide glyphs, wrapping, and hyperlinks.
GitOrigin-RevId: 7dfb737e762f7f056a38774d956f618090809a62
30ab0ee65b