Keep candidates found at a rename's revision when clearing offers

A candidate search that read a dinner after its rename committed could
store fresh offers that the rename's own clear then dropped, so a valid
candidate for the renamed dinner was refused with a 400.

clearOffers now takes the revision below which offers are stale: a rename
passes its committed revision, so it drops only offers and in-flight
searches from earlier revisions, while grounding by hand and archiving
pass math.MaxInt64 and clear everything as before. A search finishing
late at an earlier revision no longer replaces offers already found at a
later one.

Fixes MQ-103-02.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dinners/export_test.go

@@ -49,6 +49,14 @@ func (s *Service) HasOffers(id string) bool {
 	return ok
 }
 
+// StoreOffers stores urls as dinner id's offers, as a Candidates search
+// that read the dinner at grounding_revision revision would on finishing.
+func (s *Service) StoreOffers(id string, urls map[string]string, revision int64) {
+	keep, done := s.trackOffers(id)
+	defer done()
+	keep(urls, revision)
+}
+
 // OfferCount is the number of dinners whose candidates the service holds.
 func (s *Service) OfferCount() int {
 	s.offersMu.Lock()