fix(server-transfer): break loader sort ties by transferID

NewServerTransferClass merged Verified and acked-rollback rows and sorted
them by AckedAt only. On Windows time.Now() granularity is ~15.6ms, so
MarkVerified and an immediately-following MarkRevertDelivered routinely
share a timestamp. The stable sort then left the Verified candidate
ahead of the rollback that is actually on disk and the agent was locked
out on the next restart. Add transferID as a deterministic tiebreaker so
the later rotation always wins.

service/singleton/server_transfer.go

@@ -307,6 +307,7 @@ func NewServerTransferClass() *ServerTransferClass {
 
 	type credCandidate struct {
 		serverID   uint64
+		transferID uint64
 		secret     string
 		ackedAt    time.Time
 		isRevert   bool
@@ -320,6 +321,7 @@ func NewServerTransferClass() *ServerTransferClass {
 		}
 		candidates = append(candidates, credCandidate{
 			serverID:   t.ServerID,
+			transferID: t.ID,
 			secret:     t.HandshakeSecret,
 			ackedAt:    *t.AckedAt,
 			toUserID:   t.ToUserID,
@@ -332,13 +334,25 @@ func NewServerTransferClass() *ServerTransferClass {
 		}
 		candidates = append(candidates, credCandidate{
 			serverID:   t.ServerID,
+			transferID: t.ID,
 			secret:     t.RevertHandshakeSecret,
 			ackedAt:    *t.AckedAt,
 			isRevert:   true,
 			toUserID:   t.FromUserID,
 		})
 	}
+	// Sort newest-first by AckedAt, breaking ties with transferID. AckedAt
+	// alone is not enough on platforms whose time.Now() granularity is
+	// coarse (Windows: ~15.6ms): MarkVerified and the immediately-following
+	// MarkRevertDelivered routinely produce identical timestamps, and a
+	// stable sort then leaves the Verified candidate (appended first) ahead
+	// of the rollback that is actually on disk, locking the agent out on
+	// restart. transferID is monotonically increasing within a server's
+	// transfer lifecycle, so the later rotation always wins the tiebreak.
 	sort.SliceStable(candidates, func(i, j int) bool {
+		if candidates[i].ackedAt.Equal(candidates[j].ackedAt) {
+			return candidates[i].transferID > candidates[j].transferID
+		}
 		return candidates[i].ackedAt.After(candidates[j].ackedAt)
 	})