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/movement.rs

@@ -6,7 +6,7 @@ use settled_reach_server::simulation::SimulationPlugin;
 #[test]
 fn movement_validated_within_app() {
     let mut app = App::new();
-    app.add_plugins(SimulationPlugin);
+    app.add_plugins(SimulationPlugin { seed: 0 });
 
     // Insert a walkability map with one blocked tile
     let mut map = WalkabilityMap::new(10, 10, 1);
@@ -56,7 +56,7 @@ fn movement_validated_within_app() {
 #[test]
 fn entity_collision_blocks_movement() {
     let mut app = App::new();
-    app.add_plugins(SimulationPlugin);
+    app.add_plugins(SimulationPlugin { seed: 0 });
     app.insert_resource(WalkabilityMap::new(10, 10, 1));
 
     // Stationary entity at (5,4)