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/smoke.rs

@@ -7,7 +7,7 @@ use settled_reach_server::simulation::SimulationPlugin;
 #[test]
 fn world_boots_and_ticks() {
     let mut app = App::new();
-    app.add_plugins(SimulationPlugin);
+    app.add_plugins(SimulationPlugin { seed: 0 });
 
     // Verify initial state
     let time = app.world().resource::<SimulationTime>();