feat(engine): add StatsPlugin with persistent stats and toggleable overlay

StatsPlugin loads stats on startup, persists them on every GameWonEvent
and abandoned NewGameRequestEvent (>=1 move, not won), and provides a
full-window overlay toggled with `S` showing games played/won, win rate,
streak, best score, fastest win, and average win time.

The storage path is configurable via StatsPlugin::storage_path: the
default ctor uses dirs::data_dir(); StatsPlugin::headless() disables
I/O entirely so tests don't read or overwrite the user's real
stats.json. record_abandoned runs before GameMutation so it reads
move_count before handle_new_game clobbers it.

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

solitaire_engine/src/lib.rs

@@ -7,6 +7,7 @@ pub mod game_plugin;
 pub mod input_plugin;
 pub mod layout;
 pub mod resources;
+pub mod stats_plugin;
 pub mod table_plugin;
 
 pub use animation_plugin::{AnimationPlugin, CardAnim};
@@ -19,4 +20,5 @@ 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 stats_plugin::{StatsPlugin, StatsResource, StatsScreen};
 pub use table_plugin::{PileMarker, TableBackground, TablePlugin};