Closes the coverage gap the README flagged as "Next": two real str0m clients
connect to the sidecar over loopback UDP (full ICE/DTLS/SRTP + the data-channel
renegotiation the browser does), peer A sends Opus RTP, and we assert peer B
RECEIVES the forwarded media. Pure `cargo test`, ~0.8s, no browser — proves the
SFU data plane (accept offer → forward without decode) end to end.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>