Fleet Web e2e seam check failed once: tile ec2:7 error 20.5 against a limit of 3 #1348

Seen on #1346's first Fleet Web run (job 121738214837): the e2e seam check failed with tile ec2:7 at an error of 20.5 against a limit of 3. The same head passed it in every other run: three matched CI samples (run 40372934826), the measurement run (40366842617), two local runs in the CI container, and main's job after the merge (121859190116). No mechanism found yet, so this is open, not dismissed: a torn seam on the page is exactly what the sync rule exists to prevent. Next step if it recurs: the test's trace artifact, and whether ec2:7 was drawn from a different t than its neighbours at that moment (see core/sync.ts).