feat(host/windows): host→client Opus audio — vendored libopus on MSVC

The `m3` audio_thread (desktop capture → Opus 48 kHz stereo 5 ms CBR →
AUDIO_MAGIC datagrams) now runs on Windows, fed by the WASAPI loopback
capturer. The `opus` crate vendors libopus via `audiopus_sys` + cmake
(no system lib / vcpkg), so it builds on MSVC — moved into a
`cfg(any(linux, windows))` deps table and widened the audio_thread cfg
to match (the stub now only covers other targets, e.g. macOS).

Build note: CMake 4 rejects libopus's old `cmake_minimum_required`;
set `CMAKE_POLICY_VERSION_MINIMUM=3.5` when building the host on Windows.

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

crates/punktfunk-host/Cargo.toml

@@ -53,6 +53,12 @@ utoipa-scalar = { version = "0.3", features = ["axum"] }
 tower = { version = "0.5", features = ["util"] }
 http-body-util = "0.1"
 
+# Opus stereo encode for the host->client audio plane. The `opus` crate vendors libopus via
+# `audiopus_sys` (cmake-built from source — no system lib, no vcpkg), so it builds on Windows MSVC
+# too (needs CMake + NASM, both on the box). Both platforms that have an audio-capture backend.
+[target.'cfg(any(target_os = "linux", target_os = "windows"))'.dependencies]
+opus = "0.3"
+
 [target.'cfg(target_os = "linux")'.dependencies]
 # `screencast` gates the ScreenCast portal module; `remote_desktop` adds the RemoteDesktop
 # portal we use for libei input on KWin/GNOME; `tokio` is the default runtime.
@@ -81,8 +87,6 @@ wayland-backend = "0.3"
 serde_json = "1"
 # Builds/validates the xkb keymap uploaded to the virtual keyboard + tracks modifier state.
 xkbcommon = "0.8"
-# Opus encode for the GameStream audio stream (links system libopus).
-opus = "0.3"
 # The safe `opus` crate is stereo-only; surround (5.1/7.1) needs the libopus *multistream*
 # encoder (`opus_multistream_encoder_*`). `audiopus_sys` is the sys layer `opus` already
 # vendors (same libopus link), so this adds bindings, not a second copy of the library.

crates/punktfunk-host/src/m3.rs

@@ -1340,7 +1340,7 @@ fn input_thread(
 /// The audio thread: desktop capture → Opus (48 kHz stereo, 5 ms, CBR — same tuning as the
 /// GameStream path) → `AUDIO_MAGIC` datagrams. QUIC already encrypts; no extra layer.
 /// The capturer comes from (and returns to) the persistent slot — see [`AudioCapSlot`].
-#[cfg(target_os = "linux")]
+#[cfg(any(target_os = "linux", target_os = "windows"))]
 fn audio_thread(conn: quinn::Connection, stop: Arc<AtomicBool>, audio_cap: AudioCapSlot) {
     use crate::audio::{CHANNELS, SAMPLE_RATE};
     const FRAME_MS: usize = 5;
@@ -1415,11 +1415,9 @@ fn audio_thread(conn: quinn::Connection, stop: Arc<AtomicBool>, audio_cap: Audio
 
 /// Stub — punktfunk/1 audio needs Linux (PipeWire capture + libopus); non-Linux dev builds
 /// run sessions without it, same as when the capturer fails to open.
-#[cfg(not(target_os = "linux"))]
+#[cfg(not(any(target_os = "linux", target_os = "windows")))]
 fn audio_thread(_conn: quinn::Connection, _stop: Arc<AtomicBool>, _audio_cap: AudioCapSlot) {
-    tracing::warn!(
-        "punktfunk/1 audio requires Linux (PipeWire + libopus) — session continues without it"
-    );
+    tracing::warn!("punktfunk/1 audio requires Linux or Windows — session continues without it");
 }
 
 fn synthetic_stream(