Stop a clean migration reporting domains it never lost

The post-migration comparison had the two versions counting domains
differently, and yesterday's wiring turned that into a gate: `run` would have
failed a migration that lost nothing.

The 0.15 side added every domain appearing in any account's address on top of
the domain principals - the fallback's own comment says "if the instance has
no explicit domain principals", but the loop ran unconditionally. The 0.16
side did the reverse, listing only domains some account calls its primary,
discarding the full Domain list it had already fetched. An instance with
three declared domains and accounts aliased across nine reported nine before
and three after. INBUXA is exactly that shape, and this was the account/domain
over-count noted as undiagnosed.

Both sides now mean "the domains this server holds". A domain that still goes
missing is reported as a warning rather than failing the run: what the two
versions call a domain differs across this boundary in ways we have now been
caught by once, and a missing account - which is compared with a local-part
fallback and is what actually matters - still fails.

Narrowing OK() also made String() return before printing the domain lines,
so the new warning would have been silent. Caught by its own test.

README.md

@@ -39,8 +39,11 @@ was down for **6 seconds** end to end. Plan the window around verification,
 not data volume.
 
 After cutover, `run` compares the migrated instance against the snapshot
-preflight took, and fails the command if an account or a domain that existed
-before is missing from it. The service is left running either way — by that
+preflight took, and fails the command if an account that existed before is
+missing from it. A domain that no longer appears is reported as a warning
+rather than a failure: the two versions do not agree on what counts as a
+domain — principals on one side, `Domain` objects on the other — and failing
+a migration over that difference would abort runs that lost nothing. The service is left running either way — by that
 point the store has been migrated in place, so stopping it would not undo
 anything; your recovery point is the way back. `report <run-id>` prints the
 same finding again later. Where preflight had no admin URL to snapshot from,