extract the shared Centroid helper into ai/Centroid.h

src/lib/ecs/system/ai/StandbyExecutor.cpp

@@ -5,6 +5,7 @@
 #include <QVector2D>
 
 #include "BehaviorKind.h"
+#include "Centroid.h"
 #include "EntityAdmin.h"
 #include "FactionComponent.h"
 #include "HealthComponent.h"
@@ -16,28 +17,6 @@
 #include "StationBodyComponent.h"
 #include "tracing.h"
 
-namespace
-{
-// Accumulates positions to produce their centroid (the center between them).
-struct Centroid
-{
-    QVector2D sum;
-    int       count = 0;
-
-    void add(const QVector2D& point)
-    {
-        sum += point;
-        count += 1;
-    }
-
-    std::optional<QVector2D> value() const
-    {
-        if (count == 0) { return std::nullopt; }
-        return sum / static_cast<float>(count);
-    }
-};
-}
-
 void StandbyExecutor::execute(EntityAdmin& admin)
 {
     TRACE();