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/InputMapper.cpp

@@ -17,6 +17,7 @@
 #include "ModeCancelRequestedEvent.h"
 #include "PanDirectionChangedEvent.h"
 #include "PauseToggleRequestedEvent.h"
+#include "SelectionClearRequestedEvent.h"
 #include "SpeedStepRequestedEvent.h"
 #include "TemporaryBlueprintCaptureRequestedEvent.h"
 #include "TemporaryBlueprintPlaceRequestedEvent.h"
@@ -153,6 +154,10 @@ bool InputMapper::handleKeyPress(QKeyEvent* event, const ControlContext& context
         EventManager::getInstance()->sendEventImmediately(
             std::make_shared<ModeCancelRequestedEvent>());
         return true;
+    case ControlAction::ClearSelection:
+        EventManager::getInstance()->sendEventImmediately(
+            std::make_shared<SelectionClearRequestedEvent>());
+        return true;
     case ControlAction::CopyTemporary:
         // The BlueprintLibrary owns the selection and blueprint-capture logic; it drives
         // placement mode from there (REQ-UI-BLUEPRINT-TEMP).