split the selection panel into one card per kind of selection

src/ui/selection/MultiBuildingContent.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include <vector>
+
+#include "BuildingId.h"
+#include "SelectionContent.h"
+#include "SelectionContentFactory.h"
+
+class QLabel;
+
+// The count summary for several selected buildings that do not aggregate
+// (REQ-UI-MULTI-SELECTION, REQ-UI-SELECTION-AGGREGATE): how many of each type, and the
+// total building block cost of the selection. No per-building detail is shown.
+class MultiBuildingContent : public SelectionContent
+{
+    Q_OBJECT
+
+public:
+    MultiBuildingContent(const SelectionContext& context,
+                         const SelectionRequest& request, QWidget* parent = nullptr);
+
+protected:
+    // The summary is fixed for a given selection -- how many of each type were selected,
+    // and what they cost -- so it is built once and has nothing to keep current. A
+    // building leaving the selection re-publishes it and rebuilds this card.
+    void refreshRuntime() override {}
+
+private:
+    void buildSummary();
+
+    std::vector<BuildingId> m_ids;
+    QLabel*                 m_countsLabel;
+    QLabel*                 m_totalCostLabel;
+};