## Why
Lost tmux resize notifications can leave the TUI using stale terminal dimensions.
## What changed
Add a background size monitor for tmux on Unix that checks terminal dimensions every 500 ms and delivers detected changes as resize events. Queries run off the UI thread so a slow query does not block input.
Pause monitoring during terminal handoffs and discard samples invalidated by newer geometry or pause/resume transitions. Ignore unchanged, invalid, or failed samples, and stop monitoring without waiting for a blocked query.
## Testing
Add tests for missed-resize recovery and text wrapping, input responsiveness during blocked queries, stale sample rejection, pause/resume handling, nonblocking shutdown, and tmux-only activation.
GitOrigin-RevId: 461f09bf792f1d9760672dd33b125ecdf0ebd1e0
7d2c58e6e0