refactor(engine): deduplicate TABLEAU_FAN_FRAC constant (#59)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/card_plugin.rs

@@ -27,7 +27,7 @@ use crate::card_animation::CardAnimation;
 use crate::events::{CardFaceRevealedEvent, CardFlippedEvent, StateChangedEvent};
 use crate::font_plugin::FontResource;
 use crate::game_plugin::GameMutation;
-use crate::layout::{Layout, LayoutResource, LayoutSystem};
+use crate::layout::{Layout, LayoutResource, LayoutSystem, TABLEAU_FAN_FRAC};
 use crate::pause_plugin::PausedResource;
 use crate::platform::USE_TOUCH_UI_LAYOUT;
 use crate::resources::{DragState, GameStateResource};
@@ -40,11 +40,6 @@ use crate::ui_theme::{
     TYPE_BODY, Z_STOCK_BADGE,
 };
 
-/// Fraction of card height used as vertical offset between face-up tableau cards.
-/// Must match `layout::TABLEAU_FAN_FRAC` so the initial layout and the first
-/// dynamic update from `update_tableau_fan_frac` produce identical spacing.
-pub const TABLEAU_FAN_FRAC: f32 = 0.18;
-
 /// Per-card vertical step for face-down tableau cards, as a fraction of
 /// card height. Smaller than [`TABLEAU_FAN_FRAC`] because face-down cards
 /// don't need their full body shown — only the back-pattern strip is