docs(host): prove unsafe blocks in the Windows + cross-platform files + gate them (unsafe-proof program 3/N)

Continues the unsafe-proof program across the Windows/cross-platform host files
(~75 blocks, 21 files), each with a SAFETY proof of the real invariant and a
per-file #![deny(clippy::undocumented_unsafe_blocks)] gate:

  capture/windows: dxgi.rs, wgc_relay.rs, wgc.rs, desktop_watch.rs, composed_flip.rs
                   (windows-rs COM: interface validity, same-D3D11-device textures,
                    immediate-context single-thread, borrowed args outlive the call)
  windows: service.rs (SCM/token/CreateProcessAsUserW/event handles — OwnedHandle
           liveness, no double-close/signal race), win_display, wgc_helper, interactive
  vdisplay/windows: manager.rs, pf_vdisplay.rs (SwDeviceCreate/IddCx/ioctl handle
                    liveness via the OnceLock VDM singleton + OwnedHandle)
  encode/windows: ffmpeg_win.rs (full AVBufferRef refcount audit — balanced, NO leaks,
                  unlike the vaapi sibling), sw.rs
  cross-platform: gamestream/audio.rs (libopus), gamestream/stream.rs (sendmmsg),
                  inject/windows/sendinput.rs, audio/windows/wasapi_mic.rs,
                  session_tuning.rs, vdisplay.rs

Two findings (handled separately):
- wgc_relay.rs `unsafe impl Sync for HelperRelay` is UNSOUND (its mpsc Receiver is
  !Sync) though not live-exploited — marked SUSPECT inline; fix pending box check
  (it touches the in-flight punktfunk1.rs).
- capture.rs / encode.rs (PARENT modules of the WIP idd_push.rs / nvenc.rs) do NOT
  get the file deny yet — it would propagate the lint into the undocumented WIP
  children. The deny lands there once those are documented (after the WIP commits).

Linux-visible parts verified green (cargo clippy -p punktfunk-host --all-targets
-- -D warnings). The cfg(windows) deny gates are box-verified next.

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

crates/punktfunk-host/src/capture/windows/desktop_watch.rs

@@ -7,6 +7,9 @@
 //! desktop's NAME (WTS session notifications miss UAC entirely, so the name is the reliable signal)
 //! and publishes it as an atomic the capture mux + input path read.
 
+// Every `unsafe` block in this file carries a `// SAFETY:` proof; enforce it (unsafe-proof program).
+#![deny(clippy::undocumented_unsafe_blocks)]
+
 use std::sync::atomic::{AtomicBool, AtomicU8, Ordering};
 use std::sync::Arc;
 use std::time::Duration;
@@ -33,6 +36,10 @@ impl DesktopWatcher {
         // mux) sees the real state immediately. Otherwise a session that begins already on the secure
         // desktop (e.g. a reconnect to a locked box) would read DESKTOP_NORMAL for the first poll
         // interval and relay one stale normal-desktop frame — the "flash of the login screen" bug.
+        // SAFETY: `is_secure_desktop` is this module's `unsafe fn` — unsafe only because it calls Win32
+        // desktop FFI (`OpenInputDesktop`/`GetUserObjectInformationW`/`CloseDesktop`), with no caller
+        // precondition; it opens, names, and closes the input-desktop handle internally and is safe to
+        // call from any thread (here, on the thread running `DesktopWatcher::start`).
         let initial = if unsafe { is_secure_desktop() } {
             DESKTOP_SECURE
         } else {
@@ -53,6 +60,9 @@ impl DesktopWatcher {
                 let mut candidate = initial;
                 let mut stable = 0u32;
                 while !st.load(Ordering::Relaxed) {
+                    // SAFETY: same as in `start` — `is_secure_desktop` is self-contained Win32 desktop
+                    // FFI with no caller precondition, called here on the dedicated `desktop-watch`
+                    // polling thread.
                     let v = if unsafe { is_secure_desktop() } {
                         DESKTOP_SECURE
                     } else {