Membership cleanup: implement join/joining in terms of mark()

The paths are actually not so easy in there, since there's replacements and from/to to keep in mind etc.

It might be good to have only one path: `mark(node, as: )` and we always update or add a node.

`join` and `joining` can remain but should be only delegating to `mark`

Start by tracing the implementations of `join`, `joining`, and `mark`, then inspect how replacement and `from`/`to` cases are handled. Done means `join` and `joining` delegate to `mark` while preserving the existing membership-update behavior across those paths.

Written by the indexing model from the issue text.