Adversarial review F4 (Med, confirmed): spec §4.5 specifies '60 s of no
RTP packets.' The implementation bumped `last_rx` on every datagram
`Receive::new` classified — including STUN consent checks and DTLS
keepalives. A peer that kept ICE alive but sent no audio (muted mic,
revoked permission) was never reaped within 60 s.
Move the bump to `handle_event::Event::MediaData` only — only
depacketized audio resets the timer. Spec §4.5 wording was already
correct (clarified in 8680f2b); the code now matches.
TDD via the new PacketIo harness:
- FakePackets (VecDeque-backed) injects an RTP-header-shaped datagram
(classifiable but not MediaData-bearing) without a network round-trip.
- RED: `f4_non_rtp_datagram_does_not_reset_idle_timer` asserted
`last_rx == initial`; pre-fix the bump in Step 1 on every classified
datagram made the test fail.
- GREEN: removed the Step-1 bump; added the bump in
`handle_event::Event::MediaData`. Test passes.
(Fixture rationale: STUN bytes don't work — `StunMessage::parse` needs
a USERNAME attribute that doesn't exist without a real handshake, so
`Receive::new` returns Err and the bump is skipped → false-pass.
RTP-header bytes multiplex as RTP without further validation, so they
reach `handle_input` — exactly the bug shape.)
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