implement scrap and ship skeleton

src/lib/sim/Simulation.h

@@ -12,6 +12,8 @@
 #include "Tick.h"
 
 class BuildingSystem;
+class ShipSystem;
+class ScrapSystem;
 
 class Simulation
 {
@@ -36,6 +38,10 @@ public:
     const BuildingSystem& buildings() const;
     BeltSystem&           belts();
     const BeltSystem&     belts() const;
+    ShipSystem&           ships();
+    const ShipSystem&     ships() const;
+    ScrapSystem&          scraps();
+    const ScrapSystem&    scraps() const;
 
 private:
     EntityId allocateId();  // Strictly increasing; never returns kInvalidEntityId.
@@ -49,6 +55,8 @@ private:
 
     BeltSystem                      m_beltSystem;
     std::unique_ptr<BuildingSystem> m_buildingSystem;
+    std::unique_ptr<ShipSystem>     m_shipSystem;
+    std::unique_ptr<ScrapSystem>    m_scrapSystem;
 
     std::vector<FireEvent>          m_fireEvents;
     std::vector<BlueprintDropEvent> m_blueprintDropEvents;