Phase 3 of the roadmap — presence + polish.
- Peer cursors on the grid (scroll-tracked, frozen-pane-aware, zoom-aware).
- Live-typing ghost — characters appear in the peer’s cell as they type.
- Presence avatars with “Active now / Last seen Ns ago” tooltips.
- “Waiting to reconnect” banner + faster offline detection.
- Divergence detector — amber “Out of sync” pill when state vectors diverge.
- Session-history side panel — per-room op log with timestamps and revert.