Storing checked that the identity scope was resolved, but not that it
was still the one the login was made under. Credentials are filed per
identity, and the round trip leaves room for the scope to move — the
account obtained here would then be written into somebody else's
namespace and synced to their Keychain.
Only a move between two named identities counts. Unresolved at either
end is left to the storing side, which already refuses to write under a
scope it cannot name; rejecting it here as well would turn an ordinary
sign-in that began before iCloud settled — or a tokenless scope
revalidating on activation, which it does every time — into a failed
login.
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