Refactor `UserData` trait to provide bin string instead of digest

Idea is to have `UserData` trait impls provide bin string (i.e. input to hash algo) instead of the digest itself to allow for composition.

Start by locating the UserData trait and every implementation, then trace where the digest is produced and passed to the hash algorithm. Done means implementations provide the binary input string instead, with the hashing boundary supporting composition consistently across those implementations.

Written by the indexing model from the issue text.