feat(engine): add layout, LayoutResource, and TablePlugin

compute_layout is a pure function that maps window size to card size and
the 13 pile positions, with clamping at the 800x600 minimum and seven
tableau columns horizontally aligned with stock/waste (cols 0,1) and the
four foundations (cols 3,4,5,6). TablePlugin spawns a 2D camera, a felt
background sprite, and 13 translucent pile-marker sprites, and
repositions them on WindowResized. Plugin registers WindowResized
explicitly so it works under MinimalPlugins in tests.

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

solitaire_engine/src/lib.rs

@@ -6,11 +6,15 @@
 
 pub mod events;
 pub mod game_plugin;
+pub mod layout;
 pub mod resources;
+pub mod table_plugin;
 
 pub use events::{
     CardFlippedEvent, DrawRequestEvent, GameWonEvent, MoveRequestEvent, NewGameRequestEvent,
     StateChangedEvent, UndoRequestEvent,
 };
 pub use game_plugin::GamePlugin;
+pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};
+pub use table_plugin::{PileMarker, TableBackground, TablePlugin};