Resume interrupted editor co-op runs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/MULTIPLAYER.md

@@ -90,6 +90,11 @@ per turn. Fog of war, particles and the camera are local and not part of the sha
   five strongest creatures and 20% of his gold, like the single-player `Carry`. The host stores the progress
   (missions done, chosen branches, run seed) in local storage under `dc.coop_editor_progress` (campaign name ->
   `CampaignProgress`). A run ends when no next mission fits co-op.
+- **Resuming a run**: the host's progress also keeps where the run stands (`CampaignProgress::next`, set when a
+  mission starts and after a victory) and the carry into that mission. While a run is open the Match page shows
+  *Continue Mn*: with the relay it creates a room whose `create` carries `"seed"` (the run seed) and `"carry"`;
+  the relay's `start` then has that seed, the carry and `"resume":true`, and the clients treat it like `next`
+  (the progress is not reset). Without the relay the run goes on offline with the AI partner.
 - **Co-op offline with an AI partner** (*AI partner* on the Match page, works without the server): the selected
   co-op mission (built-in, the league or the editor campaign) starts like a match with the player on side 0 and
   an AI seat on side 2 (`App::start_offline_coop`: `start_match` + `sync_ai_sides`), then `end_match` drops the