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/determinism.rs

@@ -38,7 +38,7 @@ use settled_reach_server::simulation::SimulationPlugin;
 /// Snapshots are written to SnapshotBuffer for direct inspection.
 fn build_deterministic_app(seed: u64) -> App {
     let mut app = App::new();
-    app.add_plugins(SimulationPlugin);
+    app.add_plugins(SimulationPlugin { seed });
     app.add_plugins(BridgePlugin);
     app.add_plugins(KnowledgePlugin);
     app.add_plugins(NpcPlugin);