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>

Cargo.lock

@@ -226,6 +226,17 @@ version = "1.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1505bd5d3d116872e7271a6d4e16d81d0c8570876c8de68093a09ac269d8aac0"
 
+[[package]]
+name = "audiopus_sys"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62314a1546a2064e033665d658e88c620a62904be945f8147e6b16c3db9f8651"
+dependencies = [
+ "cmake",
+ "log",
+ "pkg-config",
+]
+
 [[package]]
 name = "autocfg"
 version = "1.5.1"
@@ -413,6 +424,15 @@ dependencies = [
  "generic-array",
 ]
 
+[[package]]
+name = "block-padding"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a8894febbff9f758034a5b8e12d87918f56dfc64a8e1fe757d65e29041538d93"
+dependencies = [
+ "generic-array",
+]
+
 [[package]]
 name = "bumpalo"
 version = "3.20.3"
@@ -425,6 +445,15 @@ version = "1.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1e748733b7cbc798e1434b6ac524f0c1ff2ab456fe201501e6497c8417a4fc33"
 
+[[package]]
+name = "cbc"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "26b52a9543ae338f279b96b0b9fed9c8093744685043739079ce85cd58f289a6"
+dependencies = [
+ "cipher",
+]
+
 [[package]]
 name = "cbindgen"
 version = "0.29.3"
@@ -1206,6 +1235,7 @@ version = "0.1.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "879f10e63c20629ecabbb64a8010319738c66a5cd0c29b02d63d272b03751d01"
 dependencies = [
+ "block-padding",
  "generic-array",
 ]
 
@@ -1454,11 +1484,13 @@ dependencies = [
  "ashpd",
  "axum",
  "axum-server",
+ "cbc",
  "ffmpeg-next",
  "hex",
  "libc",
  "lumen-core",
  "mdns-sd",
+ "opus",
  "pipewire",
  "rand 0.8.6",
  "rcgen",
@@ -1700,6 +1732,15 @@ version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7c87def4c32ab89d880effc9e097653c8da5d6ef28e6b539d313baaacfbafcbe"
 
+[[package]]
+name = "opus"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4d3809943dff6fbad5f0484449ea26bdb9cb7d8efdf26ed50d3c7f227f69eb5c"
+dependencies = [
+ "audiopus_sys",
+]
+
 [[package]]
 name = "ordered-stream"
 version = "0.2.0"