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/windows/interactive.rs

@@ -15,6 +15,9 @@
 //! that is correct for launching *our own* streamer, but a store launcher needs the real user's token
 //! for activation + auth). The host process itself stays SYSTEM.
 
+// Every `unsafe` block in this file carries a `// SAFETY:` proof; enforce it (unsafe-proof program).
+#![deny(clippy::undocumented_unsafe_blocks)]
+
 use anyhow::{bail, Context, Result};
 use std::path::Path;
 use windows::core::{PCWSTR, PWSTR};
@@ -40,6 +43,8 @@ use windows::Win32::System::Threading::{
 /// user is logged on (a pre-login / freshly-booted box can stream the login desktop but cannot
 /// auto-launch a store title until someone signs in).
 pub fn spawn_in_active_session(cmdline: &str, workdir: Option<&Path>) -> Result<u32> {
+    // SAFETY: `spawn_inner` is unsafe only for its Win32 FFI; it has no caller-side preconditions — it
+    // validates the session/token itself and owns every handle it opens — so calling it is always sound.
     unsafe { spawn_inner(cmdline, workdir) }
 }