relay: a close that stop() retries successfully lets the host record be cleared #828

Found by the 2026-11-07 post-merge audit of #794 and verified against main 709c161 (LOW: fails safe today because real transports do not reject a close; becomes reachable once #774 lands).

What the fix does: the relay's "unconfirmed close" flag is set only when a channel is dropped from the retained set without being closed. A retained channel that stop() then closes successfully on retry no longer marks the stop as unconfirmed, so the host record is cleared and stop() does not report a disconnect failure — whether the original rejection arrived before stop() started or while it was waiting on that close. The existing case "stop() waits for a pending reconnect close that rejects" in relay-failed-channels-refresh-783.test.ts currently expects the failure despite a successful retry; its expectation changes with the fix, and a new case covers both orderings on both close paths.

Note: open PR #819 moves the same assignment into its new close helper; build this after #819 merges.

Refs #791, #794, #774