refactor(engine,wasm,data): route all klondike/card_game imports through solitaire_core

All downstream crates now import Foundation, KlondikePile, Tableau,
Klondike, Session, Suit, Rank exclusively from solitaire_core.
solitaire_core is the single version-pin point for the upstream crates.

- solitaire_engine: 19 files updated, klondike direct dep removed
- solitaire_wasm: use statement updated, klondike direct dep removed
- solitaire_data: unused klondike dep removed
- Cargo.lock: klondike no longer a direct dep of engine/wasm/data
- Full workspace clippy clean, all tests pass

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

solitaire_engine/src/table_plugin.rs

@@ -6,7 +6,7 @@
 
 use bevy::prelude::*;
 use bevy::window::WindowResized;
-use klondike::{Foundation, KlondikePile, Tableau};
+use solitaire_core::{Foundation, KlondikePile, Tableau};
 use solitaire_core::card::Suit;
 
 use crate::events::{HintVisualEvent, StateChangedEvent};
@@ -22,15 +22,28 @@ use crate::ui_theme::TEXT_PRIMARY;
 use solitaire_data::Theme;
 
 /// Default tint applied to every empty-pile marker sprite. Pure white
-/// at 8% alpha — soft enough that the marker reads as a "hint of a
-/// slot" rather than a panel, but visible against every felt
-/// background.
+/// at 15% alpha — soft enough that the marker reads as a "hint of a
+/// slot" rather than a panel, but discernible even against a very dark
+/// felt background under bright ambient light (the old 8% alpha vanished
+/// on a #151515 felt during on-device Android testing).
 ///
 /// Re-exported as the source of truth for `cursor_plugin::MARKER_DEFAULT`,
 /// which used to duplicate the literal alongside a "kept in sync" doc
 /// comment. Pulling both call sites through this const makes drift a
 /// compile error instead of a stale comment.
-pub const PILE_MARKER_DEFAULT_COLOUR: Color = Color::srgba(1.0, 1.0, 1.0, 0.08);
+pub const PILE_MARKER_DEFAULT_COLOUR: Color = Color::srgba(1.0, 1.0, 1.0, 0.15);
+
+/// Tint applied to the thin outline rectangle sitting behind every
+/// empty-pile marker. A slightly brighter white at 28% alpha gives the
+/// slot a defined edge — the standard solitaire "empty pile" affordance —
+/// without competing with real cards. Rendered as a marginally larger
+/// child rectangle one z-step behind the fill, so the fill overlaps it
+/// and only a hairline frame remains visible.
+const PILE_MARKER_OUTLINE_COLOUR: Color = Color::srgba(1.0, 1.0, 1.0, 0.28);
+
+/// Width in logical pixels of the visible outline frame around an empty
+/// pile marker (the outline rect is this much larger on each side).
+const PILE_MARKER_OUTLINE_WIDTH: f32 = 2.0;
 
 /// Holds pre-loaded [`Handle<Image>`]s for the 5 selectable table backgrounds.
 ///
@@ -286,6 +299,22 @@ fn spawn_pile_markers(commands: &mut Commands, layout: &Layout) {
             PileMarker(pile),
         ));
 
+        // Outline frame: a marginally larger rectangle sitting one z-step
+        // behind the fill. The fill overlaps its centre, leaving only a
+        // hairline border visible — a defined slot edge without an extra
+        // asset or 9-slice. Untagged so the `PileMarker` count is unchanged.
+        let outline_size = marker_size + Vec2::splat(PILE_MARKER_OUTLINE_WIDTH * 2.0);
+        entity.with_children(|b| {
+            b.spawn((
+                Sprite {
+                    color: PILE_MARKER_OUTLINE_COLOUR,
+                    custom_size: Some(outline_size),
+                    ..default()
+                },
+                Transform::from_xyz(0.0, 0.0, -0.05),
+            ));
+        });
+
         // Tableau markers show "K" (only a King may start an empty column).
         // Foundation markers show "A" (only an Ace may claim an empty slot).
         // Neither label carries a suit because any suit may start any slot.
@@ -577,7 +606,7 @@ mod tests {
             .world_mut()
             .query::<&PileMarker>()
             .iter(app.world())
-            .map(|m| m.0.clone())
+            .map(|m| m.0)
             .collect();
         types.sort_by_key(|p| format!("{p:?}"));
         types.dedup();
@@ -607,9 +636,9 @@ mod tests {
         let mut visible_piles: Vec<KlondikePile> = Vec::new();
         for (marker, visibility) in q.iter(app.world()) {
             if matches!(visibility, Visibility::Hidden) {
-                hidden_piles.push(marker.0.clone());
+                hidden_piles.push(marker.0);
             } else {
-                visible_piles.push(marker.0.clone());
+                visible_piles.push(marker.0);
             }
         }