## Why
Context compaction was only reported after it completed, so the TUI did not
show when compaction was actively making room to continue.
## What changed
- Show `Compacting context` as soon as manual or server-driven compaction
starts, with an elapsed timer independent of the current turn timer.
- Preserve the compaction status across follow-up input and transient activity,
while still allowing retry errors to take precedence.
- Restore in-progress compaction timing from thread snapshots and add the live
duration to the completion message.
- Clear stale compaction state when the turn starts, completes, or terminates.
## Testing
Added TUI tests covering manual and server-driven compaction, narrow rendering,
follow-up input, replay and task switching, retry status, timer preservation,
and turn termination.
GitOrigin-RevId: 804e1bc553315da24ff1f99462e8e282818cada3
a526f54b00