feat(core): Step 2 — replace pile management with Session<Klondike>

- Delete rules.rs (228 lines) — move validation now handled by klondike engine
- Delete SolverState DFS from solver.rs (~900 lines) — replaced by session.solve()
- Rewrite GameState::new_with_mode() using Klondike::with_seed() (removes deck.rs dep)
- Rewrite move_cards/draw/undo to use Session<Klondike> as move executor
- Remove internal undo_stack (VecDeque<StateSnapshot>) — session owns history
- Sync piles from KlondikeState after each move via sync_piles_from_session()
- Update engine layer (game_plugin, input_plugin, card_plugin, etc.) to new API
- Net: 821 insertions, 3872 deletions (-3051 lines)

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

solitaire_engine/src/card_plugin.rs

@@ -20,7 +20,6 @@ use solitaire_core::card::{Card, Rank, Suit};
 use solitaire_core::game_state::{DrawMode, GameState};
 use solitaire_core::pile::PileType;
 
-use solitaire_core::rules::{can_place_on_foundation, can_place_on_tableau};
 
 use crate::animation_plugin::{CARD_ANIM_Z_LIFT, CardAnim, EffectiveSlideDuration};
 use crate::card_animation::CardAnimation;
@@ -1683,17 +1682,13 @@ fn handle_right_click(
         return;
     };
 
+    let Some(source_pile) = game.0.pile_containing_card(card.id) else {
+        return;
+    };
+
     // Tint piles that legally accept the card.
     for (entity, pile_marker, mut sprite) in &mut pile_markers {
-        let pile_type = &pile_marker.0;
-        let Some(pile) = game.0.piles.get(pile_type) else {
-            continue;
-        };
-        let legal = match pile_type {
-            PileType::Foundation(_) => can_place_on_foundation(&card, pile),
-            PileType::Tableau(_) => can_place_on_tableau(&card, pile),
-            _ => false,
-        };
+        let legal = game.0.can_move_cards(&source_pile, &pile_marker.0, 1);
         if legal {
             sprite.color = RIGHT_CLICK_HIGHLIGHT_COLOUR;
             commands