#28152 acpmux: a peer link hears no change of a row mirrored from another peer

Draft: red test first; the fix follows.

A daemon that mirrors its peers' chats relays every change of a mirrored row on every other peer's link, which drops it (peer.rs, the session_changed reader). With 25 peers each change went out 24 times, with its tools.

Agent session: agent session

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Summary by cubic

Adds a regression test proving a peer link hears no change of a row mirrored from another peer; the test is red on base.

The daemon relays every change of a mirrored row on every other peer's link, where peer.rs drops it. With 25 peers each change of one peer's chat went out on the 24 other links for nothing, with its tools. The fix follows.