entity per world

Game/Entity/entity_common.h

@@ -7,6 +7,12 @@ struct Class;
 using DerivedType = void*;
 using EntityID    = uint64_t;
 
+constexpr uint8 kInvalid_user_id = 0U;
+constexpr EntityID kInvalid_entity_id = 0ULL;
+
+constexpr EntityID kEntityID_user_id_shift = 56ULL;
+constexpr EntityID kEntityID_user_id_mask = 0xFF00'0000'0000'0000ULL;
+
 // clang-format off
 #define ENTITY_TYPE_LIST(X) X(Inert),