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/lib/eventsystem/event/ExpansionCostChangedEvent.h

@@ -5,8 +5,9 @@
 
 // Fired when the current asteroid-expansion cost changes (REQ-EXP-COST): once at
 // startup and again after each expansion is purchased. Carries no payload — the
-// header Expand button re-reads Simulation::getCurrentExpansionCost() to update
-// its caption and enabled state (REQ-UI-EXPAND-BUTTON).
+// expansion button re-reads Simulation::getCurrentExpansionCost() to update its cost
+// line and enabled state, and moves onto the next stretch of locked ground, the
+// buildable edge having shifted with the purchase (REQ-UI-EXPAND-BUTTON).
 class ExpansionCostChangedEvent : public Event
 {
 };