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/error_handling.rs

@@ -46,7 +46,7 @@ fn malformed_input_produces_sim_error_and_server_continues() {
         let bridge = TcpBridge::accept_on(listener).expect("accept connection");
 
         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>();
@@ -177,7 +177,7 @@ fn state_hash_populated_in_snapshot() {
         let bridge = TcpBridge::accept_on(listener).expect("accept connection");
 
         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>();