chore(engine): re-export BackgroundImageSet from engine lib

The resource is defined in table_plugin and used by the rest of the
engine, but it was the only one of the prominent table_plugin types not
re-exported from lib.rs. Add it next to PileMarker / TableBackground so
downstream binaries can reference it without reaching into the module
path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -88,7 +88,9 @@ pub use resources::{DragState, GameStateResource, HintCycleIndex, SettingsScroll
 pub use selection_plugin::{SelectionHighlight, SelectionPlugin, SelectionState};
 pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen, StatsUpdate};
 pub use sync_plugin::{SyncPlugin, SyncProviderResource};
-pub use table_plugin::{HintPileHighlight, PileMarker, TableBackground, TablePlugin};
+pub use table_plugin::{
+    BackgroundImageSet, HintPileHighlight, PileMarker, TableBackground, TablePlugin,
+};
 pub use time_attack_plugin::{
     TimeAttackEndedEvent, TimeAttackPlugin, TimeAttackResource, TIME_ATTACK_DURATION_SECS,
 };