/users package

IsNameTaken reports whether the exact-string name exists in nameStore. Returns true for any name ever registered, including:

• active registrations
• tombstoned (deleted) users' names — Delete() sets `deleted=true` but does not remove the nameStore entry (anti-revival policy)
• old aliases from renames — UpdateName inserts the new name alongside the old; the old key stays (anti-rename-squat policy)

In short: IsNameTaken(name) answers "would RegisterUser(name, _) fail with ErrNameTaken?" — same answer for active, deleted, or aliased-away names. Pairs with IsCanonicalTaken (canonical-match) and ResolveName (active-current-user lookup with full UserData).

No canonicalization is applied. For controllers that want exact- match uniqueness without pulling in canonical-collision logic.