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_core/src/lib.rs

@@ -5,6 +5,5 @@ pub mod error;
 pub mod game_state;
 pub mod klondike_adapter;
 pub mod pile;
-pub mod rules;
 pub mod scoring;
 pub mod solver;