fix Ctrl+click no longer deselecting

Regression from the SelectionController extraction. The original started a box
drag only in the empty-space branch, after the building, actor and debris hits
had each returned. Pulling the hit resolution out into selectAtPoint left the box
drag behind as unconditional, so every click started one.

That made a click on an object resolve twice: once on press, and again on release
as a 1x1 box over the same tile. For a plain click both resolutions are Replace
with the same object, so nothing looked wrong. For Ctrl+click the second
resolution toggled the object straight back on, so it never deselected - and the
selection rectangle drawn during the drag is the box that should not have been
started.

selectAtPoint now reports whether it hit anything, and only a click that hit
nothing starts a box drag.

The SelectionController itself was correct, and its tests still pass: the bug was
entirely in the widget wiring, which nothing on this branch can test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JcReq7hVk4KUPhTDKWAG7K

src/ui/GameWorldView.h

@@ -193,8 +193,9 @@ private:
     void                     pruneDespawnedActors();
     // Resolves a click into the selection it should produce, applying the category
     // precedence of REQ-UI-SELECTION-CATEGORIES; the controller owns what that then
-    // does to the existing selection.
-    void                     selectAtPoint(QPoint tile, QVector2D worldPos, bool additive);
+    // does to the existing selection. Returns false when the click hit nothing,
+    // 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);
     void                     stepSpeed(int delta);
     void                     placeAtTile(QPoint tile);