Document the co-op campaign

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

docs/MULTIPLAYER.md

@@ -119,7 +119,7 @@ per turn. Fog of war, particles and the camera are local and not part of the sha
 | C | Server: lobby and match relay over WebSocket (axum), turns, pause on disconnect, AI takeover on leave, chat, ping | built |
 | D | Client: *Multiplayer* menu (lobby, create, join, seats, ready), WebSocket bridge in `dc_plugin.js`, chat, ping | built |
 | E | Alliances and co-op rules, AI masters for free and abandoned seats | built: co-op alliance, teams A-D in versus, sending gold, fallen hearts as spectators, AI seats |
-| F | Co-op campaign: 20 missions with story (EN/DE) | partly built: co-op matches reuse the single player missions that fit the master areas (`coop_mission_indexes`); no own co-op missions or story yet |
+| F | Co-op campaign: 20 missions with story (EN/DE) | built: "League against the Lightwatch" (`src/editor/coop_campaign.rs`), an editor campaign with painted maps, branches, raids and bosses; on the Match page the co-op source cycles built-in missions → the league → the editor's campaign |
 
 | G | Editor campaigns in co-op over several missions (branches, carry, host progress), campaign names in the room list | built |