rename EntityId to BuildingId

src/lib/core/BuildingId.h

@@ -0,0 +1,9 @@
+#pragma once
+
+// Stable id for factory buildings, construction sites, and belts.
+// Ships, stations, scrap, and the HQ proxy use entt::entity instead.
+// Ids are allocated centrally by the Simulation, strictly increasing, never
+// reused. 0 is reserved as the invalid id.
+using BuildingId = long long;
+
+constexpr BuildingId kInvalidBuildingId = 0;