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/lib/eventsystem/event/CMakeLists.txt

@@ -25,6 +25,7 @@ SET(HDRS
 	${CMAKE_CURRENT_SOURCE_DIR}/SpeedStepRequestedEvent.h
 	${CMAKE_CURRENT_SOURCE_DIR}/GhostRotationRequestedEvent.h
 	${CMAKE_CURRENT_SOURCE_DIR}/ModeCancelRequestedEvent.h
+	${CMAKE_CURRENT_SOURCE_DIR}/SelectionClearRequestedEvent.h
 	${CMAKE_CURRENT_SOURCE_DIR}/DebugDrawToggleRequestedEvent.h
 	${CMAKE_CURRENT_SOURCE_DIR}/DeconstructModeChangedEvent.h
 	${CMAKE_CURRENT_SOURCE_DIR}/BuildingTypeSelectedEvent.h

src/lib/eventsystem/event/SelectionClearRequestedEvent.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "Event.h"
+
+// The player pressed the key that drops the current selection (REQ-UI-HOTKEYS). Separate
+// from ModeCancelRequestedEvent although both are Q: which of the two a press means is
+// settled by the action table, and an event that meant either would force the receiver to
+// decide it a second time.
+class SelectionClearRequestedEvent : public Event
+{
+};