stand the expansion button on the ground it buys

The button leaves the header bar for the world, centered on the columns
the next purchase unlocks and panning with them until it leaves the view.
Those columns now take a lighter tint than the locked ground behind them,
so the boundary between the two says how far one purchase reaches.

Nothing in the code reported a scroll: the camera knew it had moved and
told only the hover. A ViewScrolledEvent says it now, which is what a
widget keeping a place in the world rather than on the screen needs. The
panels want none of it -- they are placed against the screen and stay put
as the world moves under them -- so the button is placed after their pass
and never joins the rectangles they step around.

Its face is two lines, "Expand" over the cost. That makes the cost a
display of its own, so it carries the building-block tooltip the way every
other item value does, on hover alone since the click buys the expansion.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/ui/GameWorldView.h

@@ -120,6 +120,13 @@ public:
     // no blueprint is active and for the unnamed temporary one (REQ-UI-BLUEPRINT-TEMP).
     QString getActiveBlueprintName() const;
 
+    // The world <-> widget transform for the current viewport size and scroll
+    // position. Cheap to build and deliberately not cached: it is a snapshot that
+    // a resize or a scroll invalidates, so every user takes a fresh one. Public because
+    // a widget standing in the world needs it to find its place (REQ-UI-EXPAND-BUTTON);
+    // a ViewScrolledEvent says when it has gone stale.
+    WorldCoordinates getCoordinates() const;
+
 protected:
     void initializeGL() override;
     void paintGL() override;
@@ -189,11 +196,6 @@ private:
     // Gathers the interaction state the renderer needs for this frame.
     WorldRenderFrame makeRenderFrame() const;
 
-    // The world <-> widget transform for the current viewport size and scroll
-    // position. Cheap to build and deliberately not cached: it is a snapshot that
-    // a resize or a scroll invalidates, so every user takes a fresh one.
-    WorldCoordinates getCoordinates() const;
-
     float    getAsteroidLeftEdge() const;
     float    getEnemyStationRightEdge() const;
     // The camera's current pan limits, read fresh from the simulation each frame: