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.rs

@@ -2,6 +2,11 @@
 //! CPU-copy fallback (the portal delivers a CPU buffer; the encoder uploads it to the GPU
 //! internally). Zero-copy dmabuf→NVENC import is deferred (plan §9 risk).
 
+// This file's own unsafe block carries a `// SAFETY:` proof, but the file-level
+// `#![deny(clippy::undocumented_unsafe_blocks)]` is deliberately NOT set yet: as a parent module it
+// would propagate the lint to `capture::windows::idd_push` (in-flight parallel work, not yet
+// proven). The deny lands here once every child module (incl. idd_push.rs) is documented.
+
 use anyhow::Result;
 
 /// Packed pixel layout of a [`CapturedFrame`]. The ScreenCast portal negotiates the
@@ -433,6 +438,11 @@ pub fn capture_virtual_output(
     // DDA is the safety net (+ the secure-desktop path). The encode thread is set MTA so the WGC
     // objects built on the watchdog thread (also MTA) are usable here; the keepalive is handed to WGC
     // only on success, else to DDA. A hung watchdog thread is abandoned (holds no keepalive).
+    // SAFETY: `RoInitialize` is a combase FFI call that initializes the WinRT apartment for the calling
+    // thread. It takes the `RO_INIT_MULTITHREADED` enum by value and borrows no memory, so there is no
+    // pointer/lifetime/aliasing obligation; it is safe on any thread and idempotent — a second call on a
+    // thread already in a compatible apartment returns S_FALSE / RPC_E_CHANGED_MODE, which we discard.
+    // Runs on the encode thread that goes on to use the WGC (WinRT) objects built by the watchdog thread.
     unsafe {
         let _ = windows::Win32::System::WinRT::RoInitialize(
             windows::Win32::System::WinRT::RO_INIT_MULTITHREADED,