The check that stops two usernames normalizing to the same home directory
ran as a storage operation separate from the save, so two first-time users
provisioned concurrently could both observe a free scope and both be saved
into it, ending up sharing one home directory.
Move the check into users.Storage.SaveProvisioned, which holds a single
lock across the lookup and the save, and reduce CreateUserHome to deriving
and creating the directory. This covers all three provisioning paths:
signup, proxy auth and hook auth.
Refs GHSA-j7jh-37pf-mf8h
Unit tests for Settings.CreateUserHome (derives a per-user home, rejects a
colliding scope, preserves an explicit scope) plus proxy and hook regression
tests asserting that with CreateUserDir enabled two provisioned users receive
distinct home directories instead of the server root.
Refs GHSA-j7jh-37pf-mf8h, GHSA-j2fc-28fx-hc8q
