feat(engine): right-click highlight timer and visual hint glow (#5, #6)

Task #5: Add RightClickHighlightTimer(1.5 s) so destination highlights
auto-despawn after 1.5 s. Existing clear-on-state-change and
clear-on-pause logic still fires early when a move is made or the game
is paused. Three unit tests cover timer countdown behaviour.

Task #6: Add HintVisualEvent emitted on H key. Source card gets
HintHighlight + HintHighlightTimer(2 s) for a yellow glow. Destination
PileMarker gets HintPileHighlight with a gold tint (Color::srgb(1.0,
0.85, 0.1)) that restores the original colour when the 2 s timer
expires. Five unit tests cover timer expiry and colour invariants.

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

solitaire_engine/src/events.rs

@@ -102,3 +102,16 @@ pub struct XpAwardedEvent {
 /// persists stats, and starts a fresh deal.
 #[derive(Event, Debug, Clone, Copy, Default)]
 pub struct ForfeitEvent;
+
+/// Fired when the player requests a hint (H key). Carries the source card ID
+/// and destination pile for visual highlighting.
+///
+/// Consumed by `CardPlugin` (to apply `HintHighlight` on the card entity) and
+/// `TablePlugin` (to tint the destination `PileMarker` gold for 2 s).
+#[derive(Event, Debug, Clone)]
+pub struct HintVisualEvent {
+    /// The `Card::id` of the source card to be highlighted.
+    pub source_card_id: u32,
+    /// The destination pile whose `PileMarker` should be tinted gold.
+    pub dest_pile: solitaire_core::pile::PileType,
+}