stop hovering when the cursor points at no tile

A cursor resting on a panel or outside the window kept whatever it last
pointed at: the ghost, the tunnel preview, the deconstruct tint all stayed
put, because "not hovering" was not a state the build mode could hold. Make
both ghost tiles optional, clear the hover with them, and re-derive it when
the cursor comes back or a mode is entered.

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

src/ui/GameWorldView.h

@@ -226,6 +226,15 @@ private:
     // which is the only case that goes on to start a box drag.
     bool                     selectAtPoint(QPoint tile, QVector2D worldPos, bool additive);
     void                     selectInBox(bool additive);
+    // Whether the cursor points at the game world at all: it does while it is over
+    // this widget, and while a belt or box drag holds the button, which goes on
+    // following the cursor onto the floating panels and past the window edge.
+    bool                     isHoverLive() const;
+    // Re-derives the hover from wherever the cursor is now, or drops it when the
+    // cursor points at nothing (REQ-BLD-GHOST). The entry point for everything a
+    // mouse move does not cover: a scrolling view, a cursor crossing onto a panel or
+    // out of the window, and a mode just entered under a cursor that has not moved.
+    void                     refreshHover();
     // Re-resolves everything that follows from where the cursor points into the world:
     // the ghost tile and its validity, the tunnel ends, a running belt or box drag, the
     // deconstruct hover. Called for every mouse move, and once per frame while the view
@@ -320,6 +329,11 @@ private:
     // end tile closest to the cursor when snapping to a building (REQ-BLD-BELT-DRAG)
     // and to resolve the tunnel ghost sub-tile (REQ-BLD-TUNNEL-MODE).
     QVector2D                            m_cursorWorldPos;
+    // Whether the hover state currently stands for a cursor pointing at the world,
+    // so that losing it is noticed once rather than every frame. Kept here rather
+    // than asked of Qt per reader: it has to agree with what was last written to the
+    // build mode controller, not with where the cursor happens to be mid-frame.
+    bool                                 m_hoverLive = false;
 
     bool     m_debugDraw;