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/src/test_world/mod.rs

@@ -695,7 +695,7 @@ mod tests {
     #[test]
     fn gauntlet_setup_creates_expected_entities() {
         let mut app = App::new();
-        app.add_plugins(crate::simulation::SimulationPlugin);
+        app.add_plugins(crate::simulation::SimulationPlugin { seed: 0 });
         app.add_plugins(crate::knowledge::KnowledgePlugin);
         app.add_plugins(crate::npc::NpcPlugin);
 
@@ -718,7 +718,7 @@ mod tests {
     #[test]
     fn hub_center_is_walkable() {
         let mut app = App::new();
-        app.add_plugins(crate::simulation::SimulationPlugin);
+        app.add_plugins(crate::simulation::SimulationPlugin { seed: 0 });
         app.add_plugins(crate::knowledge::KnowledgePlugin);
         app.add_plugins(crate::npc::NpcPlugin);
 
@@ -735,7 +735,7 @@ mod tests {
     #[test]
     fn occlusion_north_wall_blocks() {
         let mut app = App::new();
-        app.add_plugins(crate::simulation::SimulationPlugin);
+        app.add_plugins(crate::simulation::SimulationPlugin { seed: 0 });
         app.add_plugins(crate::knowledge::KnowledgePlugin);
         app.add_plugins(crate::npc::NpcPlugin);
 
@@ -754,7 +754,7 @@ mod tests {
     #[test]
     fn corridor_connects_hub_to_occlusion() {
         let mut app = App::new();
-        app.add_plugins(crate::simulation::SimulationPlugin);
+        app.add_plugins(crate::simulation::SimulationPlugin { seed: 0 });
         app.add_plugins(crate::knowledge::KnowledgePlugin);
         app.add_plugins(crate::npc::NpcPlugin);
 
@@ -771,7 +771,7 @@ mod tests {
     #[test]
     fn stable_id_ranges_match_spec() {
         let mut app = App::new();
-        app.add_plugins(crate::simulation::SimulationPlugin);
+        app.add_plugins(crate::simulation::SimulationPlugin { seed: 0 });
         app.add_plugins(crate::knowledge::KnowledgePlugin);
         app.add_plugins(crate::npc::NpcPlugin);