The idle check ran only in on_frame and only for the peer that just
sent, so a peer that went quiet, or never sent (contact-only), held its
slot forever: eight spoofed addresses locked out every newcomer
(review S1).
Peers now record when their contact started (added_at), and idle peers
(now - last_inbound.unwrap_or(added_at) > PEER_IDLE_MS) are swept as if
Lost before a peer is added and at the top of next_frame. A peer that
resumes after its idle window is an implicit new contact: its frame is
processed rather than silently dropped. A repeat Contact restarts the
idle clock.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>