Replaces synchronous disk writes on the UI thread with a dedicated
persistence actor task. The UI now try_sends a PersistRequest and
returns immediately — keyboard input is never gated on write completion.
Changes:
- New persistence_actor module with bounded-coalescing actor
- Actor spawns at TUI startup; global singleton so no App struct change
- All persist_checkpoint/persist_session_snapshot/clear_checkpoint calls
replaced with persistence_actor::persist(PersistRequest::...)
- Dropped redundant TurnStarted persist (nothing changed between
SendMessage's checkpoint and TurnStarted)
- Fixed collapsible_if clippy lint
This is the P0 fix for the post-send terminal freeze caused by
serialising 500KB+ sessions to disk on the UI thread.
Hunter Bown
f1f601c28b