Co-op offline with an AI partner

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

docs/MULTIPLAYER.md

@@ -90,6 +90,13 @@ 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.
+- **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
+  turn clock and the world runs on this computer like a single-player game. The partner (`Game::is_partner`) is
+  an allied rival-style master with his own dungeon: two of three healthy creatures follow the player's attack
+  flag (and attack the gate next to it), and every minute he sends 400 gold to an ally below 300. An editor
+  campaign run goes on locally: the next mission (or the same after a defeat) with the run seed and the carry.
 - **Teams in versus**: on the Match page every seat has a team A-D (default: a team of its own), set with
   `{"t":"set_team","team":n}` for the own seat; the host may add `"side"` for a free or AI seat. Only before the
   start. Room updates carry `team` per seat, the room list `teams`, and `start` a `teams` list in the order of