## Why
Large command streams, including commands that never emit a newline, can make an active TUI cell grow without bound.
## What changed
- Retain live output in full up to 1 MiB, then preserve the first and last 50 completed lines plus the current partial line.
- Bound individual long lines while preserving UTF-8 boundaries, visible head and tail content, and ANSI rendering around omission markers.
- Show omitted line and byte counts in previews and transcripts. Final command output still replaces the bounded live preview when execution completes.
## Testing
Added unit and snapshot coverage for chunk boundaries, split CRLF sequences, newline-free and UTF-8 output, truncated ANSI sequences, retained-output limits, and preview and transcript rendering.
GitOrigin-RevId: 3cf3cc862db686f20ac09b1f6e7f9dc6d264e6eb
ef6b597f41