keep the selection panel half a tile off what it describes

The panel had one distance for the view edges, the widgets it steps around,
and the selection alike, so it stood eight pixels from a building and touched
it outright along the top. The gap from the selection is now its own value,
half a tile, and it is horizontal only -- the top edges stay level.

It is sampled where the tile size is known, in the same moment as the anchor
rectangle, and travels with it: a rectangle frozen in one moment has no
meaningful distance to a tile size measured in another. chooseSide now takes
the gap in place of the margin, the band having already taken the margin off.

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

src/lib/eventsystem/event/SelectionAnchorChangedEvent.h

@@ -15,11 +15,17 @@
 // is while the selection grows; and because the rectangle is screen space frozen at that
 // moment, scrolling the view or a selected ship flying off does not move the panel
 // either.
+//
+// The gap the panel keeps from that rectangle travels with it, for the same reason: it is
+// half a tile as the tile stood in this moment (REQ-UI-SELECTION-PANEL, REQ-GW-TILE-SIZE)
+// and stays that for as long as the selection lasts, a rectangle frozen in one moment
+// having no meaningful distance to a tile size measured in another.
 class SelectionAnchorChangedEvent : public Event
 {
 public:
-    explicit SelectionAnchorChangedEvent(QRect rectPx)
-        : rectPx(rectPx) {}
+    SelectionAnchorChangedEvent(QRect rectPx, int selectionGapPx)
+        : rectPx(rectPx), selectionGapPx(selectionGapPx) {}
 
     const QRect rectPx;
+    const int   selectionGapPx;
 };