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/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(