make construction its own system (extracted from BuildingSystem)

src/lib/sim/ConstructionSystem.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "BeltSystem.h"
+#include "FactoryState.h"
+#include "GameConfig.h"
+#include "Tick.h"
+
+// Advances the construction queue and turns a finished site into an operational
+// building (REQ-BLD-CONSTRUCTION). One site is built at a time, in queue order:
+// the front site's timer runs, and when it elapses the site becomes a Building —
+// its ports and buffers are derived from its definition, its belt tile is handed
+// back to BeltSystem, and the next queued site starts.
+//
+// It completes the building itself rather than handing the finished site back to
+// BuildingSystem: everything materialisation needs is either in FactoryState, the
+// config, or a free function (see BuildingBuffers.h, PortGeometry.h), so there is
+// no intermediate value to pass and no ordering rule between two calls.
+//
+// Holds only the config; the world it works on arrives per tick, like the other
+// systems in lib/ecs/system.
+class ConstructionSystem
+{
+public:
+    explicit ConstructionSystem(const GameConfig& config) : m_config(config) {}
+
+    void tick(FactoryState& state, BeltSystem& belts, Tick currentTick);
+
+private:
+    const GameConfig& m_config;
+};