Match loop on the client side: orders go to the server, a confirmed turn steps the world five times, and a checksum of the shared state catches worlds that diverge

docs/MULTIPLAYER.md

@@ -44,6 +44,11 @@ per turn. Fog of war, particles and the camera are local and not part of the sha
   Particles, effects and the camera keep running in real time: they are local and not part of the shared state.
 - **Who speaks**: messages, hints and interface sounds belong to the master who is being processed
   (`Game::speaking`); what another master orders never shows up on my screen.
+- **Turns and checksums** (`netplay.rs`): `Game::start_match(sides, local)` starts a match,
+  `Game::take_orders()` hands the local orders to the server, `Game::receive_turn(cmds)` applies a confirmed
+  turn (five steps) and `Game::checksum()` compares the worlds of the clients - a peer with a different number
+  is reported once (`net.desync`). A client without the next turn stands still: that is the "pause while
+  missing" rule, and the tests run two clients of one match side by side.
 - Savegames, tests and the standalone file are unchanged: `cargo test --release` covers the command path
   (including "the same actions on both paths build the same world").
 
@@ -58,9 +63,8 @@ per turn. Fog of war, particles and the camera are local and not part of the sha
 | E | Alliances and coop rules, AI masters for free and abandoned slots | planned |
 | F | Coop campaign: 20 missions with story (EN/DE) | planned |
 
-Still open on the way to C: the turn/turn-boundary client loop, the state checksum, and heroes and
-alliances for the new sides (today the hero gate path always leads to side 0's core, and 2/3 are the
-rivals' sides in free play).
+Still open on the way to C: heroes and alliances for the new sides (today the hero gate path always leads to
+side 0's core, and in free play 2 and 3 are the rivals' sides), and what a match leaves out of savegames.
 
 ## Masters and sides