A peer that is killed, logged out or rebooted sends no reset over UDP, so a
session only learns of it by timing out. Nothing here acts on that; these are
the two observations a caller needs to.
`rx_progress` counts arriving fragments. A message reaches the caller only once
it is whole, so a large one - a clipboard image is the case that occurs - leaves
the receive loop with nothing to observe for as long as the transfer takes, and
a peer sending steadily looks exactly like one that died. A counter, not a
timestamp: `Instant::now()` costs an order of magnitude more per read, and
callers only compare successive samples.
`webrtc_disconnected` exposes the ICE state webrtc-rs already raises ~5s after
it stops hearing from the peer. It is deliberately not a fourth
`WebRTCConnectionState`: that enum gates send and receive readiness and session
cache admissibility, and a transient state must change neither.
TCP and WebSocket answer for neither, on purpose: this change leaves their
existing close, error and timeout behaviour exactly as it was.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns
