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/vdisplay.rs

@@ -13,6 +13,9 @@
 //! owned keepalive whose `Drop` releases the output (RAII — no explicit `destroy`). Capture
 //! consumes the node via [`crate::capture::capture_virtual_output`].
 
+// Every `unsafe` block in this file carries a `// SAFETY:` proof; enforce it (unsafe-proof program).
+#![deny(clippy::undocumented_unsafe_blocks)]
+
 use anyhow::Result;
 pub use punktfunk_core::Mode;
 #[cfg(target_os = "linux")]
@@ -225,6 +228,8 @@ pub fn compositor_for_kind(kind: ActiveKind) -> Option<Compositor> {
 #[cfg(target_os = "linux")]
 fn default_runtime_dir() -> String {
     std::env::var("XDG_RUNTIME_DIR").unwrap_or_else(|_| {
+        // SAFETY: `getuid()` is a parameterless POSIX call that always succeeds and touches no
+        // memory — it just returns the calling process's real uid. Nothing is aliased or freed.
         let uid = unsafe { libc::getuid() };
         format!("/run/user/{uid}")
     })
@@ -245,6 +250,8 @@ fn default_bus(runtime: &str) -> String {
 #[cfg(target_os = "linux")]
 pub fn detect_active_session() -> ActiveSession {
     use std::os::unix::fs::MetadataExt;
+    // SAFETY: `getuid()` is a parameterless POSIX call that always succeeds and touches no memory —
+    // it just returns the calling process's real uid. Nothing is aliased or freed.
     let uid = unsafe { libc::getuid() };
     let xdg_runtime_dir = default_runtime_dir();
     let dbus = default_bus(&xdg_runtime_dir);