feat(host/windows): DesktopWatcher (secure-desktop detection) — step 1 of the two-process build

Polls the input-desktop name (OpenInputDesktop + GetUserObjectInformationW(UOI_NAME)) on its own
thread → Default/Winlogon atomic; the authoritative normal-vs-secure signal for the capture mux +
input path (WTS notifications miss UAC). Not yet wired into the mux (needs the SYSTEM host + WGC
helper, steps 3-5 in docs/windows-secure-desktop.md). NOTE: detecting the secure desktop requires the
host to run as SYSTEM (a user-token process can't OpenInputDesktop the Winlogon desktop).

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

crates/punktfunk-host/src/capture.rs

@@ -320,6 +320,8 @@ pub fn capture_virtual_output(_vout: crate::vdisplay::VirtualOutput) -> Result<B
     anyhow::bail!("virtual-output capture requires Linux or Windows")
 }
 
+#[cfg(target_os = "windows")]
+pub mod desktop_watch;
 #[cfg(target_os = "windows")]
 pub mod dxgi;
 #[cfg(target_os = "linux")]