Set up basic deterministic lockstep

src/game_server/RemoteGameServerHost.cpp

@@ -20,6 +20,10 @@ void RemoteGameServerHost::initializeCampaign(
 		CampaignRegistry::instance().find(campaignId);
 	if (!campaign)
 		throw std::runtime_error("Unknown remote campaign '" + campaignId + "'");
+	client_.expectCompatibility(
+		campaign->networkCompatibility.empty()
+			? "lockstep-v2:" + campaignId
+			: campaign->networkCompatibility);
 	client_.replica_.startCampaign(*campaign, zipFile, true, false);
 }
 
@@ -38,6 +42,15 @@ void RemoteGameServerHost::resetCampaign(
 
 bool RemoteGameServerHost::prepareLocation(const LocationConfig& config)
 {
+	auto existing = client_.replica_.state_.locations.find(config.id);
+	if (existing != client_.replica_.state_.locations.end() &&
+		existing->second.definition) {
+		// The location was prepared before initial_state. Loading the
+		// authoritative snapshot overlays mutable state while retaining this
+		// definition; preparing it again would reset the assigned player's
+		// position to LocationConfig::playerPosition.
+		return false;
+	}
 	return client_.replica_.prepareLocation(config);
 }
 
@@ -92,11 +105,12 @@ void RemoteGameServerHost::bindRuntimeLocation(
 void RemoteGameServerHost::update(std::int64_t deltaMs)
 {
 	client_.update(deltaMs);
-	const auto state = client_.snapshot();
-	if (!state || state->currentLocationId.empty()) return;
-	auto runtime = runtimeLocations_.find(state->currentLocationId);
-	if (runtime == runtimeLocations_.end() || !runtime->second) return;
-	runtime->second->update(deltaMs, LocationSimulationEvents{});
+	for (auto& completed : client_.takeCompletedSimulationTicks()) {
+		if (completed.locationId.empty()) continue;
+		auto runtime = runtimeLocations_.find(completed.locationId);
+		if (runtime == runtimeLocations_.end() || !runtime->second) continue;
+		runtime->second->update(kSimulationTickMs, completed.events);
+	}
 }
 
 void RemoteGameServerHost::save(nlohmann::json&)