feat: M2 P1.6 — audio (Opus + AES-CBC) and steady-rate video pacing

A stock Moonlight client now gets video + full input + AUDIO from the
from-scratch GameStream host (verified live end-to-end on a macOS client).

Audio (audio.rs, audio/linux.rs, gamestream/audio.rs):
- Capture the default PipeWire sink's monitor (system output) as interleaved
  f32 stereo @ 48kHz via stream.capture.sink, on its own thread.
- Opus-encode 5ms/240-sample stereo frames (RESTRICTED_LOWDELAY, CBR) and send
  as GameStream RTP audio: 12-byte BE RTP_PACKET (packetType 97, seq+1/pkt,
  timestamp += packetDuration, ssrc 0) on UDP 48000, after learning the client
  endpoint from its port-learning ping.
- Encrypt the Opus payload with AES-128-CBC (PKCS7), key = launch rikey, IV =
  BE32(rikeyid + seq) in [0..4]. Like the control stream, modern Moonlight
  always decrypts audio regardless of the negotiated flags — plaintext makes it
  log "Failed to decrypt audio packet" and play silence (diagnosed from the
  client log). RTP header stays in the clear. Scheme cross-checked against
  Sunshine stream.cpp/crypto.cpp + moonlight AudioStream.c.
- Pace each frame to its 5ms slot (PipeWire delivers ~1024-frame buffers) to
  avoid bursts the client's jitter buffer hears as glitches. LUMEN_AUDIO_GAIN
  applies optional linear gain for quiet sources.
- DESCRIBE SDP advertises the stereo Opus config (a=fmtp:97 surround-params).

Video (stream.rs): pace at a steady ≤60fps, re-encoding the last captured frame
when the compositor produces none. wlroots only emits on damage, so a static or
slow-updating desktop previously starved the client into a "network too slow"
abort; an unchanged frame costs a near-empty P-frame. Adds a non-blocking
Capturer::try_latest (portal drains to the freshest queued frame).

Misc: serialize pipewire init across the video + audio capture threads
(pwinit.rs, std::sync::Once) to avoid a concurrent pw_init race. Deps: opus,
cbc; libopus-dev in bootstrap-ubuntu.sh.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/lumen-host/src/audio.rs

@@ -0,0 +1,32 @@
+//! Desktop audio capture for the GameStream audio stream. On Linux: a PipeWire stream that
+//! records the default sink's monitor (i.e. everything playing out of the system), delivered
+//! as interleaved `f32` stereo PCM at 48 kHz. The audio data plane (`gamestream::audio`)
+//! reframes this into fixed Opus frames, encodes, and sends it.
+
+use anyhow::Result;
+
+/// Opus/GameStream audio is 48 kHz stereo.
+pub const SAMPLE_RATE: u32 = 48_000;
+pub const CHANNELS: usize = 2;
+
+/// Produces interleaved `f32` stereo PCM (L,R,L,R,…) at [`SAMPLE_RATE`]. Lives on its own
+/// thread; never blocks the capture loop (drops if the consumer falls behind).
+pub trait AudioCapturer: Send {
+    /// Block until the next chunk of interleaved samples is available (variable size). The
+    /// caller reframes into fixed Opus frames.
+    fn next_chunk(&mut self) -> Result<Vec<f32>>;
+}
+
+/// Open a live capturer for the default sink monitor (system output) via PipeWire.
+#[cfg(target_os = "linux")]
+pub fn open_audio_capture() -> Result<Box<dyn AudioCapturer>> {
+    linux::PwAudioCapturer::open().map(|c| Box::new(c) as Box<dyn AudioCapturer>)
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn open_audio_capture() -> Result<Box<dyn AudioCapturer>> {
+    anyhow::bail!("audio capture requires Linux + PipeWire")
+}
+
+#[cfg(target_os = "linux")]
+mod linux;