perf(engine): route remaining drag card→entity lookups through CardEntityIndex

Replace O(n) `Query::iter().find()` card scans with O(1) `CardEntityIndex`
lookups in the mouse and touch drag pipelines (`follow_drag`, `end_drag`,
`touch_follow_drag`, `touch_end_drag`) and `update_drag_shadow` — 7 sites
across 5 systems. Each ran per dragged card per frame during a drag.

`InputPlugin` now defensively `init_resource::<CardEntityIndex>()` (idempotent;
`CardPlugin` still owns and rebuilds it) so the plugin is self-sufficient in
tests. The lone remaining card-keyed `.find` is a `#[cfg(test)]` world-query
helper, which is the correct pattern there.

Completes the CardEntityIndex migration started in ef1efdc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

solitaire_engine/src/card_plugin.rs

@@ -1487,6 +1487,7 @@ fn update_drag_shadow(
     drag: Res<DragState>,
     layout: Option<Res<LayoutResource>>,
     card_entities: Query<(&CardEntity, &Transform)>,
+    card_index: Res<CardEntityIndex>,
     mut shadow: Local<Option<Entity>>,
 ) {
     if drag.is_idle() {
@@ -1503,9 +1504,9 @@ fn update_drag_shadow(
 
     // Find the world position of the first (top) dragged card.
     let top_pos = drag.cards.first().and_then(|first_card| {
-        card_entities
-            .iter()
-            .find(|(marker, _)| marker.card == *first_card)
+        card_index
+            .get(first_card)
+            .and_then(|entity| card_entities.get(entity).ok())
             .map(|(_, t)| t.translation)
     });