Change where clietn ephemeral is send to server in suggested steps

I noticed that step 2 suggests sending the client's public ephemeral value to the server, but the server only makes use of the username. We can probably defer sending that information until it is actually used by the server in step 4 (e.g. send it along with the proof in step 3).

Start by tracing the implementation of suggested steps 2 through 4 and where the server consumes the username and client's public ephemeral value. Confirm the current message flow, then verify that the ephemeral value is deferred until it is sent with the proof in step 3 and used by the server in step 4.

Written by the indexing model from the issue text.