feat(engine): add InputPlugin with keyboard and stock-click

Keyboard: U=undo, N=new game, D=draw, Escape=pause placeholder (logged
only until the pause screen lands). Mouse: left-click on the stock pile
fires DrawRequestEvent. Cursor coordinates are converted via the active
Camera2d's viewport_to_world_2d so the hit-test works under arbitrary
camera setups.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -7,6 +7,7 @@
 pub mod card_plugin;
 pub mod events;
 pub mod game_plugin;
+pub mod input_plugin;
 pub mod layout;
 pub mod resources;
 pub mod table_plugin;
@@ -17,6 +18,7 @@ pub use events::{
     StateChangedEvent, UndoRequestEvent,
 };
 pub use game_plugin::{GameMutation, GamePlugin};
+pub use input_plugin::InputPlugin;
 pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};
 pub use table_plugin::{PileMarker, TableBackground, TablePlugin};