feat(engine): branded splash screen on launch

The window previously snapped straight to a card deal, which read more
like a prototype than a finished game. SplashPlugin lays a fullscreen
overlay (BG_BASE backdrop, ACCENT_PRIMARY title, version subtitle) on
top of the gameplay layer for MOTION_SPLASH_TOTAL_SECS — the board
deals behind it so the splash dissolve hands off naturally to the
deal animation.

Visibility curves through fade-in (300ms), hold (~1s), fade-out
(300ms) using a pure splash_alpha helper that gets pinned by a unit
test rather than wired to the Bevy clock — Time<Virtual>'s 250ms
per-tick clamp makes float-tight alpha assertions around the fade
boundary brittle.

Any keystroke or mouse-button press jumps the age forward to the
fade-out window so the splash dissolves immediately. The dismiss
handler is read-only on ButtonInput / Touches, so the same press is
still visible to gameplay handlers downstream — pressing Space on the
splash both dismisses it and triggers the next-tick stock draw, as
verified by dismissal_keypress_is_visible_to_other_systems.

Z_SPLASH sits above every other UI rung (Z_TOAST + 100) so the splash
owns the viewport for its brief lifetime. The hierarchy test was
extended to enforce the new rung's monotonic position.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_app/src/main.rs

@@ -10,8 +10,8 @@ use solitaire_engine::{
     CardPlugin, ChallengePlugin, CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin,
     FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
     OnboardingPlugin, PausePlugin, ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin,
-    StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, UiFocusPlugin, UiModalPlugin,
-    UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
+    SplashPlugin, StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, UiFocusPlugin,
+    UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
 fn main() {
@@ -101,6 +101,7 @@ fn main() {
         .add_plugins(UiModalPlugin)
         .add_plugins(UiFocusPlugin)
         .add_plugins(UiTooltipPlugin)
+        .add_plugins(SplashPlugin)
         .run();
 }