perf: Insertion-sort the sweep broadphase and swap-remove its active list

packages/flame/lib/src/collisions/broadphase/sweep/sweep.dart

@@ -17,7 +17,21 @@ class Sweep<T extends Hitbox<T>> extends Broadphase<T> {
 
   @override
   void update() {
-    items.sort((a, b) => a.aabb.min.x.compareTo(b.aabb.min.x));
+    // Between two ticks the hitboxes only move a little, so [items] is
+    // always nearly sorted: an insertion sort runs in close to linear time
+    // here, where a general-purpose sort would pay its full O(n log n) on
+    // every tick. This also avoids allocating a comparator closure per tick.
+    final items = this.items;
+    for (var i = 1; i < items.length; i++) {
+      final item = items[i];
+      final minX = item.aabb.min.x;
+      var previousIndex = i - 1;
+      while (previousIndex >= 0 && items[previousIndex].aabb.min.x > minX) {
+        items[previousIndex + 1] = items[previousIndex];
+        previousIndex--;
+      }
+      items[previousIndex + 1] = item;
+    }
   }
 
   @override
@@ -44,7 +58,10 @@ class Sweep<T extends Hitbox<T>> extends Broadphase<T> {
             _prospectPool.acquire(item, activeItem);
           }
         } else {
-          _active.remove(activeItem);
+          // The order of the active list does not matter, so the removal can
+          // swap in the last element instead of searching and shifting.
+          _active[i] = _active.last;
+          _active.removeLast();
         }
       }
       _active.add(item);