float the selection panel over the game world instead of a side column

src/ui/BuildButtonBar.h

@@ -51,6 +51,11 @@ public:
     // re-center later driven by an unlock needs no second call from the owner.
     void anchorTo(const QRect& worldViewRect);
 
+    // Height of the strip the bar occupies along the bottom of the world view: its own
+    // height plus the margin below it. The selection panel keeps out of this strip, and
+    // the bar never moves for the panel in return (REQ-UI-BUILD-BAR).
+    int getStripHeightPx() const;
+
     void clearActiveButton();
 
 private: