null if not found.
Looks up a member by key and returns a pointer to it.
Lifetime warning: the returned pointer points inside the node's internal member storage. Appending members (or any operation that reallocates the storage, such as addMember/setMembers) invalidates previously returned pointers - writes through stale pointers are silently lost. Finish all member appends before taking pointers, or re-fetch the pointer after mutations.