fix(rtp): tolerate muxed RTCP on RTP ingress

[ryanmurf]

Summary

• demux RTCP immediately after UDP receive and before RTP source/framing/PT/SSRC hygiene
• add an isolated Chime-style socket harness covering discrete RTP+1 RTCP plus SR/RR interleaved with accepted PCMU on the RTP port
• prove Chime-style a=rtcp and a=rtcp-mux SDP parses and answers without rejection

Captured RED

Before the production demux was added:

rtp::tests::chime_rtcp_does_not_disturb_rtp_ingress ... FAILED
assertion `left == right` failed
  left: 1
 right: 0

The Sender Report was entering the non-negotiated-RTP-PT counter (and the short RR took malformed-RTP handling), despite real RTP continuing around it.

GREEN

• cargo test -p asterisk-sip chime_ -- --nocapture: 2 passed
• cargo test -p asterisk-sip: 812 unit + 5 parser regression + 8 URI edge + 1 doc passed
• cargo test --workspace --exclude pjsip-shim: passed
• cargo clippy --workspace --exclude pjsip-shim -- -D warnings: passed
• cargo build --workspace: passed

All commands used Rust 1.97.0 and CARGO_BUILD_JOBS=6. No live stack, pods, Helm, trunk, or Docker was used.

Pre-M9 gate

Muxed RTCP is now discarded before RTP ingress hygiene without changing the remote address, SSRC latch, accepted media counters, or discard counters. Separate-port RTCP is independent of the RTP socket. The active SIP channel path does not instantiate the existing RTCP SR builder, so rustisk still emits no Sender Reports; that is report-only and does not block call audio.

The M9 direct-Chime call will not break on RTCP.