feat(host/windows): client→host mic passthrough via a virtual audio device

The host received the client's mic uplink (0xCB Opus) but dropped it on Windows ("requires
Linux"). Windows has no user-mode way to CREATE a capture endpoint, so target an existing
virtual audio device and write the decoded mic PCM into its RENDER endpoint — the device's
CAPTURE endpoint then surfaces as a microphone host apps record from (the inverse of a
virtual cable). New audio::wasapi_mic::WasapiVirtualMic: finds the device by friendly-name
(Steam Streaming Microphone / VB-Audio CABLE Input / VoiceMeeter / "virtual", override with
PUNKTFUNK_MIC_DEVICE), opens a WASAPI shared event-driven RENDER client (48 kHz stereo f32,
autoconvert), and a dedicated COM thread writes a bounded (~80 ms drop-oldest) inject queue
with silence-fill. open_virtual_mic() gets a Windows arm; mic_service_thread (Opus decode →
push) now compiles for windows too (opus is already a windows dep). Clear error + install
guidance when no virtual device is present.

Linux/cross-platform side cargo-checks; the Windows path is built/validated when the box is
back (the wasapi render API was cross-checked against the docs + the existing capture path).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/audio.rs

@@ -67,18 +67,27 @@ pub trait VirtualMic: Send {
     }
 }
 
-/// Open a virtual microphone PipeWire source with `channels` interleaved channels (1 or 2).
+/// Open a virtual microphone with `channels` interleaved channels (1 or 2). Linux: a PipeWire
+/// `Audio/Source`. Windows: writes into an existing virtual audio device's render endpoint (whose
+/// capture endpoint apps see as a mic) — see [`wasapi_mic`].
 #[cfg(target_os = "linux")]
 pub fn open_virtual_mic(channels: u32) -> Result<Box<dyn VirtualMic>> {
     linux::PwMicSource::open(channels).map(|m| Box::new(m) as Box<dyn VirtualMic>)
 }
 
-#[cfg(not(target_os = "linux"))]
+#[cfg(target_os = "windows")]
+pub fn open_virtual_mic(channels: u32) -> Result<Box<dyn VirtualMic>> {
+    wasapi_mic::WasapiVirtualMic::open(channels).map(|m| Box::new(m) as Box<dyn VirtualMic>)
+}
+
+#[cfg(not(any(target_os = "linux", target_os = "windows")))]
 pub fn open_virtual_mic(_channels: u32) -> Result<Box<dyn VirtualMic>> {
-    anyhow::bail!("virtual mic requires Linux + PipeWire")
+    anyhow::bail!("virtual mic requires Linux + PipeWire or Windows + a virtual audio device")
 }
 
 #[cfg(target_os = "linux")]
 mod linux;
 #[cfg(target_os = "windows")]
 mod wasapi_cap;
+#[cfg(target_os = "windows")]
+mod wasapi_mic;