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/simulation/economy_plugin.rs

@@ -8,13 +8,17 @@ use bevy_ecs::schedule::IntoScheduleConfigs;
 
 use crate::tick_phases::TickPhase;
 
-pub struct EconomyPlugin;
+pub struct EconomyPlugin {
+    /// World seed from `StartupMessage` (#826). Passed to `try_load_economy` so the
+    /// tâtonnement simulation is seeded deterministically from the client's world seed.
+    pub seed: u64,
+}
 
 impl Plugin for EconomyPlugin {
     fn build(&self, app: &mut App) {
         // Economy simulation (#821, D-031) — loaded once at startup, no-op when DB absent.
-        // Uses seed 0 for now; will be threaded through StartupMessage world seed (#826).
-        if let Some((econ_sim, econ_state)) = super::economy::try_load_economy(0) {
+        // Seed comes from StartupMessage.world_seed, threaded via SimulationPlugin (#826).
+        if let Some((econ_sim, econ_state)) = super::economy::try_load_economy(self.seed) {
             app.insert_resource(econ_sim).insert_resource(econ_state);
         }
         // EconQueryBuffer: always registered so EconStateQuery PlayerActions are accepted