v0.8.0

**Theme: Opus codec support.** SiphonAI previously advertised only G.711/G.722 and rejected Opus at config load. Opus is now negotiable.

Opus runs at a **16 kHz bridge rate** — `opus/48000/2` on the wire, but the WS path sees a 16 kHz session (`start.audio.sample_rate = 16000`), so the fixed 8/16 kHz PCM16 audio contract is unchanged and the **WS protocol stays `version: "1"`**. The RTP clock stays 48 kHz per RFC 7587; only the WS-facing PCM is 16 kHz. libopus handles the 48↔16 resample and stereo→mono downmix internally.

**Off by default** — add `"opus"` to `[media].codecs`.

### Highlights
- **Opus negotiation** end-to-end: a peer offering `opus/48000/2` (or an outbound route listing `"opus"`) now negotiates Opus; the dynamic payload type is preserved on the answer (RFC 3264).
- **First native build dependency: libopus** (via `audiopus_sys`, built from source) — building now needs a C toolchain + CMake; the shipped Dockerfile already has them. Runtime image unaffected. This is why it's a minor bump.
- **forge-media pin bumped** to add the 16 kHz Opus bridge rate (forge-media#75, mirroring G.722's wire-clock-vs-PCM-rate split).
- **SIPp `opus` regression phase** asserts the 200 OK answers Opus and the call comes up at 16 kHz.

### Notes
- **SDP `fmtp` (`stereo=0` / `useinbandfec` / `maxplaybackrate`) is a follow-up** — Opus is correct without it; the params want real-device validation.

PRs: #184 (design) → #185 (enable) → #186 (harness + docs + release).

🤖 Generated with [Claude Code](https://claude.com/claude-code)