feat(engine): add CardPlugin with procedural card rendering

Each card is a parent Sprite (white for face-up, blue for face-down) with
a Text2d child showing rank+suit (e.g. "AH", "10C", "KS"). Hearts and
diamonds render red; clubs and spades black. Face-down labels are hidden.
Tableau cards fan downward; other piles stack at the same position with a
small z-offset.

Sync runs in PostStartup for the initial deal and in Update after every
StateChangedEvent. To avoid a one-frame lag, downstream plugins run after
the new GameMutation system set.

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

solitaire_engine/src/lib.rs

@@ -4,17 +4,19 @@
 //! Additional plugins (`TablePlugin`, `CardPlugin`, `InputPlugin`,
 //! `AnimationPlugin`, etc.) land in later sub-phases of Phase 3.
 
+pub mod card_plugin;
 pub mod events;
 pub mod game_plugin;
 pub mod layout;
 pub mod resources;
 pub mod table_plugin;
 
+pub use card_plugin::{CardEntity, CardLabel, CardPlugin};
 pub use events::{
     CardFlippedEvent, DrawRequestEvent, GameWonEvent, MoveRequestEvent, NewGameRequestEvent,
     StateChangedEvent, UndoRequestEvent,
 };
-pub use game_plugin::GamePlugin;
+pub use game_plugin::{GameMutation, GamePlugin};
 pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};
 pub use table_plugin::{PileMarker, TableBackground, TablePlugin};