Outgoing voice was gated on the observer's roster copy of the remote user's
voice state, which is signaling gossip. The signal server broadcasts
`user_left` for any socket it declares dead, so a suspended laptop or a flaky
hop wiped that copy and the observer detached its microphone from a peer that
never left the channel - a silent member with no way back through the UI.
`decideVoicePathRouting` now closes a path only on positive evidence: we left
voice, the peer itself reported another channel or none, or the connection is
gone. Missing gossip holds an established path instead. Opening still needs
confirmation, so a guess never starts sending; the same rule gates playback,
camera video, and the microphone a new connection puts in its first offer.
Peers are also asked for their voice state when a connection or data channel
comes up, so a rebuilt path re-confirms itself.
Alongside it, the microphone can be switched mid-call: capture moves to a
device service and rules, the live track is swapped with `replaceTrack` so
the session is never renegotiated, and the speaking indicator follows the new
stream.
