clear the selection with Q, and drop it on entering a build mode

Implements the requirements committed in 731b887.

Q becomes a three-way branch in the action table, which is the layer that owns
what an input does: ExitMode while a mode is active, the new ClearSelection
while something is selected, EnterDeconstruct otherwise. The three partition
the situations between them, so resolution stays first-match-wins over
available actions and the handler never re-derives the precedence -- which is
why ClearSelection gets its own event rather than joining ModeCancel on Q.

GameWorldView clears the selection at each of the three events that enter a
mode; those are the only ways in, whichever button or key the player used. No
clear is needed where ModeCancel falls through to deconstruct mode: Q resolves
to ClearSelection while anything is selected, so there is nothing left by then.

The Selection context's Q row reads "Clear selection", sits last as the row
that hands the context back does everywhere, and carries the exit badge
styling -- one key that backs out should look the same wherever it appears.
Requirements follow that last point in REQ-UI-CONTROLS-CARD and
REQ-UI-CONTROLS-CONTENT.

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

src/ui/GameWorldView.h

@@ -37,6 +37,7 @@
 #include "ModeCancelRequestedEvent.h"
 #include "PanDirectionChangedEvent.h"
 #include "PauseToggleRequestedEvent.h"
+#include "SelectionClearRequestedEvent.h"
 #include "SpeedStepRequestedEvent.h"
 #include "DebugDrawToggledEvent.h"
 #include "ArtifactCountChangedEvent.h"
@@ -83,6 +84,7 @@ class GameWorldView : public QOpenGLWidget,
                                                   SpeedStepRequestedEvent,
                                                   GhostRotationRequestedEvent,
                                                   ModeCancelRequestedEvent,
+                                                  SelectionClearRequestedEvent,
                                                   DebugDrawToggleRequestedEvent,
                                                   CommandRequestedEvent>
 {
@@ -149,6 +151,7 @@ private:
     void handleEvent(std::shared_ptr<const SpeedStepRequestedEvent> event) override;
     void handleEvent(std::shared_ptr<const GhostRotationRequestedEvent> event) override;
     void handleEvent(std::shared_ptr<const ModeCancelRequestedEvent> event) override;
+    void handleEvent(std::shared_ptr<const SelectionClearRequestedEvent> event) override;
     void handleEvent(std::shared_ptr<const DebugDrawToggleRequestedEvent> event) override;
     void handleEvent(std::shared_ptr<const CommandRequestedEvent> event) override;
 
@@ -226,6 +229,12 @@ 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);
+    // A build mode and a selection are mutually exclusive, so entering any mode drops
+    // the selection (REQ-UI-SELECTION-EXCLUSIVE). Called from each of the three events
+    // that enter a mode -- the only ways in, whichever button or key the player used.
+    // Clearing an empty selection publishes nothing, so the exiting half of the
+    // deconstruct toggle costs nothing.
+    void                     clearSelectionForBuildMode();
     // 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.