CVE-2026-61544: libp2p-quic Remote Panic via Certificate Expiry Race

# Attacker-side steps (no special tooling needed beyond a custom libp2p node):
#
# 1. Generate a libp2p identity keypair.
# 2. Issue a self-signed libp2p TLS certificate with a very short validity window,
#    e.g. not_before = now, not_after = now + 2s.
# 3. Open a QUIC connection to the target libp2p listener and present this certificate.
# 4. Complete the TLS 1.3 handshake up to (but not including) the final client
#    Finished fragment. Hold that packet.
# 5. Wait until the certificate's not_after timestamp has elapsed.
# 6. Send the withheld final Finished fragment.
#
# Result: the listener's TLS layer accepts the handshake (cert was valid at parse time),
# Quinn signals handshake completion, libp2p-quic re-parses the certificate in the
# post-handshake upgrade path, libp2p_tls::certificate::parse() now sees an expired cert,
# and the .expect("peer certificate is valid") call panics, crashing the process.
#
# Relevant panicking code (pre-patch, connecting.rs L65-66):
#   let peer_id = certificate::parse(&peer_cert)
#       .expect("peer certificate is valid");