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_engine/src/lib.rs

@@ -26,6 +26,7 @@ pub mod settings_plugin;
 pub mod progress_plugin;
 pub mod resources;
 pub mod selection_plugin;
+pub mod splash_plugin;
 pub mod stats_plugin;
 pub mod sync_plugin;
 pub mod table_plugin;
@@ -97,6 +98,7 @@ pub use settings_plugin::{
 pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, HintCycleIndex, SettingsScrollPos, SyncStatus, SyncStatusResource};
 pub use selection_plugin::{SelectionHighlight, SelectionPlugin, SelectionState};
+pub use splash_plugin::{SplashAge, SplashPlugin, SplashRoot};
 pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen, StatsUpdate};
 pub use sync_plugin::{SyncPlugin, SyncProviderResource};
 pub use ui_focus::{Disabled, FocusGroup, Focusable, FocusedButton, UiFocusPlugin};