Co-op matches of editor campaigns

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

docs/MULTIPLAYER.md

@@ -69,6 +69,17 @@ per turn. Fog of war, particles and the camera are local and not part of the sha
   by the simplified rival AI on every client, so they stay deterministic.
 - **Co-op** (`Game::coop`): the master sides are allies against the heroes; allied creatures never target each
   other, allied ground does not block claims. Co-op campaign matches use `campaign::start_coop_mission`.
+- **Editor campaigns in co-op**: on the Match page the host can switch from the built-in campaign to the
+  campaign last opened in the editor (`Menu::campaign_doc`) and pick one of its missions that fits co-op
+  (`campaign::is_editor_coop_mission`: map at least 72x72, a goal besides rival masters). The create message
+  carries the whole campaign (`"doc"`, the `CampaignDoc` JSON, up to `MAX_WS_MESSAGE` = 40 MB per WebSocket
+  message); the relay keeps it in the room and adds it to the `start` message (room updates only show
+  `"editor": true`). Every client builds the mission with `campaign::start_editor_coop_mission(doc, i, seed)` from
+  the room's shared seed (rival masters of the map and the Rivals goal are left out, like in built-in co-op),
+  then the usual co-op setup. Block scripts run from side 0 in the fixed step. When the mission ends, the match
+  ends and the players go back to the lobby; there is no branching to a next mission and no progress is stored.
+- **Shared simulation side**: the shared part of a step (waves, units, towers, rivals, dwarves) runs with the
+  first master switched in on every client; only the end check and the fog use the local master.
 
 ## Steps