Fixed
- TUI freezes with 40+ sessions: Parallel status polling replaces sequential loop that couldn't complete within 2s tick
- 10-worker pool via errgroup for concurrent tmux status checks
- Instance-level RWMutex prevents data races between background worker and TUI rendering
- Tiered polling skips idle sessions with no activity (10s recheck gate)
- 3-second timeout on CapturePane/GetWindowActivity prevents hung tmux calls from blocking workers
- Timeout preserves previous status instead of flashing RED
- Race detector (-race) enabled in tests and CI