feat(ui): wire setup_egui_style one-shot into EguiPrimaryContextPass

Local<bool> guard retries until ctx_mut() first succeeds, applies the
sci-fi theme once, then never runs again. .chain()'d before ui_system so
the very first successful frame is already themed. Per-frame set_style
would dirty layout caches; this avoids that.

src/render/plugin.rs

@@ -131,7 +131,12 @@ impl Plugin for BlackHolePlugin {
             .add_systems(Update, rebuild_bloom)
             // bevy_egui 0.41 requires UI systems to run inside the egui context
             // pass (fonts/ctx are initialized there); placing them in Update panics.
-            .add_systems(bevy_egui::EguiPrimaryContextPass, crate::ui::ui_system);
+            // setup_egui_style is a one-shot (Local<bool> guard): retries until the
+            // context exists, applies the theme once, never runs again.
+            .add_systems(
+                bevy_egui::EguiPrimaryContextPass,
+                (crate::ui::setup_egui_style, crate::ui::ui_system).chain(),
+            );
     }
 }
 

src/ui/mod.rs

@@ -4,6 +4,23 @@ pub mod preset;
 use bevy::prelude::*;
 use bevy_egui::egui;
 
+/// One-shot egui styling. Registered in `EguiPrimaryContextPass` with a
+/// `Local<bool>` guard so it retries until `ctx_mut()` first succeeds, then
+/// runs exactly once. Per-frame set_style/set_visuals would dirty layout
+/// caches every frame; this avoids that.
+pub fn setup_egui_style(
+    mut contexts: bevy_egui::EguiContexts,
+    mut done: Local<bool>,
+) {
+    if *done {
+        return;
+    }
+    if let Ok(ctx) = contexts.ctx_mut() {
+        crate::ui::style::setup(&ctx);
+        *done = true;
+    }
+}
+
 pub fn ui_system(
     mut contexts: bevy_egui::EguiContexts,
     mut params: ResMut<crate::params::BlackHoleParams>,