Multiplayer plan: deterministic lockstep, steps A-F

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

docs/MULTIPLAYER.md

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+# Multiplayer - plan and status
+
+Multiplayer is only available for users of a server (not in the standalone file).
+
+## Decisions
+
+- Modes: **versus** (every player a master with an own heart), **coop** (every player an own master,
+  together against AI masters and/or the humans) and a **coop campaign of 20 missions**.
+- 2 to 4 players per match; free slots can be AI masters.
+- The match **pauses while a player is missing** (connection lost); a player who **leaves** is replaced by an AI.
+- The humans (hero waves of the Order) as a third party (can be switched off), chat, alliances/teams.
+- Coop is "separate but helpful": own heart, gold, mana, research and creatures; allies never fight each
+  other, their creatures help each other, gold can be sent, allied ground can be walked; a fallen heart only
+  knocks out that player (who may keep watching); the allies win when all enemies have fallen.
+- Lobby: an open list of matches under *Multiplayer* in the title menu; the host chooses mode, map, AI slots,
+  humans on/off and teams; logged-in users join; the match starts when everybody is ready.
+
+## Architecture: deterministic lockstep
+
+Every client runs the same simulation (the same WASM module, so floating point results are identical).
+The server does not simulate; it relays:
+
+1. Clients send their **commands** (dig, build, cast, hand, orders, ...) to the server.
+2. Every 100 ms the server sends **turn N**: the commands of all players for that turn.
+3. Each client applies turn N and simulates two fixed steps of 50 ms. Turns arrive in order; a client waits
+   for the next turn, so a missing player stalls everybody - exactly the "pause while missing" rule.
+4. Clients send a **checksum** of the game state every few seconds; a mismatch (desync) is reported.
+
+Only the commands travel over the network, so bandwidth is tiny and no game state has to be serialised
+per turn. Fog of war, particles and the camera are local and not part of the shared state.
+
+## Steps
+
+| # | Step | Status |
+|---|---|---|
+| A | Simulation per master: gold, mana, research, rooms, hand, flag, traps, payday... per side; up to four masters (red, green, blue, yellow) | planned |
+| B | All player actions as commands (`Command` + `Game::apply`), single player uses the same path; fixed time step | planned |
+| C | Server: lobby and match relay over WebSocket (axum), turns, pause on disconnect, AI takeover on leave, chat | planned |
+| D | Client: *Multiplayer* menu (lobby, create, join, ready, teams), WebSocket bridge in `dc_plugin.js`, chat, gold sending | planned |
+| E | Alliances and coop rules, AI masters for free and abandoned slots | planned |
+| F | Coop campaign: 20 missions with story (EN/DE) | planned |