Add deconstruction queue

src/lib/sim/Command.h

@@ -28,6 +28,7 @@ enum class CommandKind
 {
     PlaceBuilding,
     Demolish,
+    CancelDeconstruction,
     RotateInPlace,
     SetRecipe,
     SetShipLayout,
@@ -72,12 +73,22 @@ struct PlaceBuildingCommand : Command
     std::vector<ItemType>           splitterFilterB;
 };
 
+// Marks a building for demolition: a construction site is removed instantly, a
+// built building is queued for deconstruction (REQ-BLD-DEMOLISH, REQ-BLD-DECON-QUEUE).
 struct DemolishCommand : Command
 {
     DemolishCommand() : Command(CommandKind::Demolish) {}
     std::optional<BuildingId> id;
 };
 
+// Takes a building back out of the deconstruction queue (REQ-BLD-DEMOLISH-CLICK,
+// REQ-BLD-DEMOLISH-BOX). No-op if the id is not currently queued.
+struct CancelDeconstructionCommand : Command
+{
+    CancelDeconstructionCommand() : Command(CommandKind::CancelDeconstruction) {}
+    std::optional<BuildingId> id;
+};
+
 struct RotateInPlaceCommand : Command
 {
     RotateInPlaceCommand() : Command(CommandKind::RotateInPlace) {}