fix(onboarding): delay first-run modal until splash screen despawns

OnboardingPlugin previously used PostStartup which fires before the
first Update tick — guaranteeing the onboarding modal and the launch
splash (MOTION_SPLASH_TOTAL_SECS = 1.6 s) overlap for the entire
splash duration. The splash sits at Z_SPLASH (the highest UI z-index),
so the two screens fought visually and the user saw a confusing frozen
composite before the splash faded out.

Fix: move spawn_if_first_run to Update and gate it on
`splashes.is_empty()` (no SplashRoot entity alive). A Local<bool>
ensures the spawn fires at most once per session. Cost: ~one frame of
latency after the splash clears, which is imperceptible.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/onboarding_plugin.rs

@@ -36,6 +36,7 @@ use crate::ui_theme::{
     BORDER_SUBTLE, HighContrastBorder, RADIUS_SM, TEXT_PRIMARY, TYPE_BODY, TYPE_CAPTION,
     VAL_SPACE_1, VAL_SPACE_2, VAL_SPACE_3,
 };
+use crate::splash_plugin::SplashRoot;
 use crate::ui_theme::{TEXT_SECONDARY, Z_ONBOARDING};
 
 // ---------------------------------------------------------------------------
@@ -153,7 +154,7 @@ pub struct OnboardingPlugin;
 impl Plugin for OnboardingPlugin {
     fn build(&self, app: &mut App) {
         app.init_resource::<OnboardingSlideIndex>()
-            .add_systems(PostStartup, spawn_if_first_run)
+            .add_systems(Update, spawn_if_first_run)
             .add_systems(
                 Update,
                 (handle_onboarding_buttons, handle_onboarding_keyboard).chain(),
@@ -170,11 +171,30 @@ fn spawn_if_first_run(
     settings: Option<Res<SettingsResource>>,
     font_res: Option<Res<FontResource>>,
     mut slide_index: ResMut<OnboardingSlideIndex>,
+    splashes: Query<(), With<SplashRoot>>,
+    existing: Query<(), With<OnboardingScreen>>,
+    mut spawned: Local<bool>,
 ) {
-    let Some(s) = settings else { return };
-    if s.0.first_run_complete {
+    if *spawned {
         return;
     }
+    // Wait until the launch splash has despawned so the two screens
+    // never overlap. PostStartup would fire before the first Update
+    // tick, guaranteeing overlap; checking here costs one frame of
+    // latency after the splash clears, which is imperceptible.
+    if !splashes.is_empty() {
+        return;
+    }
+    if !existing.is_empty() {
+        *spawned = true;
+        return;
+    }
+    let Some(s) = settings else { return };
+    if s.0.first_run_complete {
+        *spawned = true;
+        return;
+    }
+    *spawned = true;
     slide_index.0 = 0;
     spawn_slide(&mut commands, 0, font_res.as_deref());
 }