feat!: Opus SDP negotiation with pinned re-offers

[wavekat-eason]

Summary

Teaches wavekat-sip's SDP layer to negotiate Opus wideband alongside G.711, with real RFC 3264 answer intersection and codec-pinned re-offers so a mid-call re-INVITE can't renegotiate under a live audio path. This is the wavekat-sip half of the WaveKat Voice Opus work (voice doc 45); it pairs with wavekat-core 0.0.13 (the opus feature) and lands second in the publish order.

The crate stays codec-agnostic — it advertises and resolves payload types and clocks; the actual encode/decode lives in wavekat-core. No new dependency.

What changes

• build_sdp_offer advertises the full menu Opus-first (opus/48000/2 at PT 111 with useinbandfec=1), G.711 after, and telephone-event at both the 8 kHz and 48 kHz clocks.
• Answers are a real RFC 3264 intersection — IncomingCall::accept picks Opus wherever the offer carries it (at the offer's PT), else the first G.711, echoes only that one selection, and answers 488 when nothing matches.
• Pinned re-offers — every post-negotiation re-offer (hold/resume, consumer-built refresh bodies) pins the negotiated codec via CodecMenu::Pinned, so a re-INVITE can't swap the codec while audio is flowing.
• parse_sdp resolves dynamic payload types into RemoteMedia::codec / opus_payload_type and records telephone-event entries at any clock (DtmfSpec); RemoteMedia::dtmf returns the clock-matched entry.
• DtmfBurstConfig.clock_rate — burst duration ticks and pacing follow the negotiated event clock (160/20 ms at 8 kHz, 960 at 48 kHz).

BREAKING CHANGE → releases as 0.2.0

• build_sdp / build_sdp_with are replaced by build_sdp_offer and a CodecMenu-taking build_sdp_with.
• RemoteMedia::dtmf_payload_type is replaced by dtmf_events / dtmf(); RemoteMedia gains codec / opus_payload_type.
• DtmfBurstConfig gains a required clock_rate field.

Test plan

• cargo test -p wavekat-sip — 229 lib tests + integration + doctests pass. cargo fmt --all -- --check clean.
• End-to-end loopback (tests/end_to_end_call.rs) negotiates Opus over real UDP sockets; a raw G.711-only INVITE exercises the legacy intersection path (and the 488-on-no-match path is covered).
• docs/RFC-COVERAGE.md updated for the new negotiation/answer behavior.

Notes for review

• Draft pending the cross-repo publish sequence: wavekat-core 0.0.13 is already published; this publishes next as 0.2.0 (breaking), then WaveKat Voice drops its local [patch.crates-io] and pins the released versions.
• One socket/timing lib test flaked once under heavy parallel load during local runs, then passed 5× clean in isolation — CI is authoritative; flag if it recurs there.

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