feat(host/windows): GameStream (Moonlight) audio on Windows — stereo

`serve` gave Moonlight clients no audio on Windows: the GameStream audio
stream thread was Linux-only (a non-Linux stub errored). Widen the
stereo path to Windows — the encode/RTP/AES-CBC/hand-rolled-RS(4,2)-FEC
logic is platform-neutral and already live-validated byte-identical on
Linux, and it now runs over the WASAPI capturer + the (already
cross-platform) `opus` crate. The cfg gates go from `linux` to
`any(linux, windows)`; only the surround path stays Linux-only because
its libopus *multistream* encoder needs `audiopus_sys` (a Linux dep) —
on Windows a surround request fails cleanly with a "use stereo" error.

Linux stays byte-identical (the `SessionEncoder::Surround` variant and
its match arm keep `#[cfg(linux)]`, so Linux compiles exactly as before).
Verified: clippy -D warnings + host test suite green on both
x86_64-pc-windows-msvc (73/73) and Linux (78/78).

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

docs/windows-host.md

@@ -29,6 +29,7 @@ Every OS-touching backend is implemented behind the existing traits and **builds
 | Run host (serve/m3-host) | ✅ live | m3-host starts + listens; `c_abi_connection_roundtrip` passes |
 | Gamepad (ViGEm) | ✅ done | compiles incl. rumble back-channel; live needs ViGEmBus + a physical pad |
 | Host→client audio wiring | ✅ done | builds on MSVC; `m3` `audio_thread` active on Windows (silent VM → no samples to send) |
+| GameStream (Moonlight) audio | ✅ done | stereo path active on Windows (WASAPI→Opus→RTP/FEC); surround stays Linux-only (libopus multistream / `audiopus_sys`) |
 | Rumble back-channel (ViGEm) | ✅ done | `request_notification` → background thread → 0xCA; live needs a physical pad |
 
 **Remaining for full parity:**