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/core/ControlAction.cpp

@@ -31,7 +31,9 @@ struct MouseBindingEntry
 
 // Resolution is first-match-wins over these tables, so an entry that must beat another
 // on the same input is listed above it -- CancelBeltLine before ExitMode on the right
-// mouse button. Everything else is disjoint by availability.
+// mouse button. Everything else is disjoint by availability: Q carries three actions
+// whose availability rules partition the situations between them, so their order here is
+// the order the reader meets them and nothing more (REQ-UI-HOTKEYS).
 const KeyBindingEntry KEY_BINDINGS[] = {
     {ControlAction::Move,             Qt::Key_A,      Qt::NoModifier},
     {ControlAction::Move,             Qt::Key_D,      Qt::NoModifier},
@@ -48,8 +50,9 @@ const KeyBindingEntry KEY_BINDINGS[] = {
     // nothing and is what puts "R" and "Shift+R" on the row.
     {ControlAction::Rotate,           Qt::Key_R,      Qt::NoModifier},
     {ControlAction::Rotate,           Qt::Key_R,      Qt::ShiftModifier},
-    {ControlAction::EnterDeconstruct, Qt::Key_Q,      Qt::NoModifier},
     {ControlAction::ExitMode,         Qt::Key_Q,      Qt::NoModifier},
+    {ControlAction::ClearSelection,   Qt::Key_Q,      Qt::NoModifier},
+    {ControlAction::EnterDeconstruct, Qt::Key_Q,      Qt::NoModifier},
     {ControlAction::OpenMenu,         Qt::Key_Escape, Qt::NoModifier},
 };
 
@@ -114,9 +117,21 @@ bool isControlActionAvailable(ControlAction action, const ControlContext& contex
     case ControlAction::SelectArea:
     case ControlAction::AddToSelection:
     case ControlAction::AddAreaToSelection:
-    case ControlAction::EnterDeconstruct:
         return context.mode == BuildMode::None;
 
+    // The three cases of Q, in the order REQ-UI-HOTKEYS evaluates them: it leaves the
+    // active mode, else clears the selection, else enters deconstruct mode. A selection
+    // and a build mode never coexist (REQ-UI-SELECTION-EXCLUSIVE), so the first two are
+    // already disjoint; what the empty-selection condition below adds is the third step,
+    // which is why entering deconstruct mode by key takes two presses while something is
+    // selected -- the Deconstruct button still gets there in one click.
+    case ControlAction::ClearSelection:
+        return context.mode == BuildMode::None
+               && context.selection != ControlSelection::None;
+    case ControlAction::EnterDeconstruct:
+        return context.mode == BuildMode::None
+               && context.selection == ControlSelection::None;
+
     // Both need something a blueprint can be made of; a selection of ships or debris
     // leaves them inert (REQ-UI-HOTKEYS).
     case ControlAction::CopyTemporary:
@@ -209,13 +224,16 @@ std::vector<ControlAction> getContextActions(const ControlContext& context)
         return filterAvailable({ControlAction::ToggleDeconstruct,
                                 ControlAction::DeconstructArea, ControlAction::ExitMode},
                                context);
+    // ClearSelection is listed last for the same reason ExitMode is above: the row that
+    // backs the player out of the context sits at the bottom of the context's rows
+    // wherever there is one (REQ-UI-CONTROLS-CONTENT).
     case ControlContextKind::Selection:
         return filterAvailable({ControlAction::Select, ControlAction::SelectArea,
                                 ControlAction::AddToSelection,
                                 ControlAction::AddAreaToSelection,
-                                ControlAction::EnterDeconstruct,
                                 ControlAction::CopyTemporary,
-                                ControlAction::CreateBlueprint},
+                                ControlAction::CreateBlueprint,
+                                ControlAction::ClearSelection},
                                context);
     case ControlContextKind::General:
         break;