feat(simulation): thread world seed from StartupMessage into economy (#826)

Replaces the hardcoded seed=0 with the seed received in StartupMessage,
threading it through SimulationPlugin -> EconomyPlugin / SimRng. Integration
test fixtures updated for the new SimulationPlugin { seed } signature.

server/tests/game_loop.rs

@@ -28,7 +28,7 @@ fn player_moves_north_through_full_pipeline() {
 
         // Build app
         let mut app = App::new();
-        app.add_plugins(SimulationPlugin);
+        app.add_plugins(SimulationPlugin { seed: 0 });
         app.add_plugins(BridgePlugin);
         app.add_plugins(KnowledgePlugin);
         app.init_resource::<TrustEventQueue>();