Do not call it data loss when the instance would not show us the accounts

The dress rehearsal reported MISSING ACCOUNT [email protected] after a
migration that lost nothing: SMTP on the migrated instance accepts mail for
it, while an address that does not exist is refused, so the account is there.

Enumeration is permission-scoped. The "before" snapshot was read by an
account with full rights; the "after" snapshot by one whose admin role the
migration had not carried across - proven one step earlier, where the quota
rebuild was refused as unauthorised. It was shown a fraction of the accounts
and the comparison called the remainder lost.

"Missing" and "not permitted to see" are different findings and only one of
them is about the data. When the migrated instance shows fewer accounts than
existed, the result now says COULD NOT VERIFY and names the reader, rather
than asserting a loss the evidence does not support. It still fails the run:
an unverified migration is not a verified one, and the fix is to re-check
with an admin account, which --resume makes cheap.

Counting accounts is not a strong enough signal to tell the two cases apart,
so it does not try to - it reports the ambiguity instead of guessing.

Also fixes the test fake, which ignored ?types= and returned domains as
individuals, making every account count in these tests wrong.

internal/validate/live.go

@@ -72,6 +72,18 @@ func RunLive(ctx context.Context, store *checkpoint.Store, rs *checkpoint.RunSta
 			return checkpoint.StepOutcome{}, err
 		}
 		switch {
+		case r.Inconclusive():
+			// Fewer accounts came back than existed, so "missing" cannot be
+			// told apart from "not permitted to see". Still a failure - an
+			// unverified migration is not a verified one - but it must not
+			// be reported as data loss, which is a different claim and one
+			// this evidence does not support.
+			return checkpoint.StepOutcome{Verdict: string(StatusFail), Detail: fmt.Sprintf(
+				"COULD NOT VERIFY (not the same as data loss): the migrated instance showed %d of %d account(s) to %s. "+
+					"Either those accounts are gone, or this account cannot see them - enumeration is permission-scoped, and a "+
+					"migration does not always carry an admin role across. Re-check with an account that holds admin on the "+
+					"migrated instance before concluding either. Findings: %s",
+				r.AccountsVisibleAfter, r.AccountsChecked, opts.AdminUser, r.String())}, nil
 		case !r.OK():
 			// Recorded as a completed step with a failing verdict rather
 			// than an error: the comparison ran, and its answer is the