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/main.rs

@@ -147,7 +147,7 @@ fn main() {
 
     let mut app = App::new();
     settled_reach_server::tick_phases::TickPhase::configure(&mut app);
-    app.add_plugins(SimulationPlugin);
+    app.add_plugins(SimulationPlugin { seed });
     app.add_plugins(BridgePlugin);
     app.add_plugins(settled_reach_server::knowledge::KnowledgePlugin);
     app.add_plugins(settled_reach_server::npc::NpcPlugin);
@@ -176,7 +176,8 @@ fn main() {
     app.insert_resource(BridgeResource::new(bridge));
     app.insert_resource(HandshakeState::Complete);
 
-    // Override SimRng with the chosen seed (SimulationPlugin defaults to seed 0)
+    // SimulationPlugin { seed } already inserts SimRng with the correct seed.
+    // Re-insert here as a defensive override in case plugin ordering shifts.
     app.insert_resource(settled_reach_server::simulation::rng::SimRng::new(seed));
 
     // Initialize empty line pool index (populated by generator pipeline in v0.2).
@@ -359,7 +360,7 @@ fn dump_schedule_graph() {
 
     let mut app = App::new();
     settled_reach_server::tick_phases::TickPhase::configure(&mut app);
-    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);