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/mod.rs

@@ -50,7 +50,12 @@ pub mod zone;
 /// Each sub-plugin owns its domain's systems, resources, and phase assignment.
 /// No system registration happens here — only sub-plugin composition and
 /// shared resources needed by multiple sub-plugins.
-pub struct SimulationPlugin;
+///
+/// `seed` is threaded through to [`economy_plugin::EconomyPlugin`] so the
+/// economy simulation uses the world seed from `StartupMessage` (#826).
+pub struct SimulationPlugin {
+    pub seed: u64,
+}
 
 impl Plugin for SimulationPlugin {
     fn build(&self, app: &mut App) {
@@ -59,7 +64,7 @@ impl Plugin for SimulationPlugin {
 
         // Shared resources needed by multiple sub-plugins.
         // Each sub-plugin inits its own domain-specific resources.
-        app.insert_resource(rng::SimRng::new(0))
+        app.insert_resource(rng::SimRng::new(self.seed))
             .init_resource::<save_io::SaveLoadPending>()
             .init_resource::<crate::knowledge::EntityRegistry>()
             // Triangle escalation event queue (#250) — consumed by storyteller + npc plugins
@@ -81,7 +86,7 @@ impl Plugin for SimulationPlugin {
         app.add_plugins(input_plugin::InputPlugin);
         app.add_plugins(movement_plugin::MovementPlugin);
         app.add_plugins(social_plugin::SocialPlugin);
-        app.add_plugins(economy_plugin::EconomyPlugin);
+        app.add_plugins(economy_plugin::EconomyPlugin { seed: self.seed });
 
         // Background worker tick integration (#843 Part C)
         app.add_systems(