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/perf_bench.rs

@@ -66,7 +66,7 @@ fn perf_tick_timing() {
         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(settled_reach_server::knowledge::KnowledgePlugin);
         app.add_plugins(settled_reach_server::npc::NpcPlugin);