rosterCleared was a single boolean latched the first time the peer set fell
below two connections. When the sole connection a was replaced by c between
ticks (still length 1) the flag stayed set, so c's gateway never received
the empty-roster push and kept the stale roster. Track the id of the
connection that got the clear instead and push whenever the sole
connection's id differs; reset once two or more connections relay again.
Review finding: routes [a] → tick → routes [c] → tick pushed no roster clear to c.
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