Regression coverage for #68634. The indicator family mounts only on the
thread's any-role tail (ba756333): a running bubble that is not the tail
stays silent, even when only a user or system row trails it, and the
optimistic-placeholder flow renders exactly one status row, the
placeholder's own.
Mutation-checked: relaxing the mount gate to a last-assistant walk fails
the two silence cases, and removing the gate fails all five.
Sora-bluesky
89fa829802