fix(windows-client): clear clippy -D warnings on MSVC

The cfg(windows) code can't be lint-checked on the Linux dev box, so three
-D warnings slipped through (caught by windows.yml; the FFI + shaders compiled
fine):
- gpu.rs: SetMultithreadProtected returns a must-use BOOL -> `let _ =`.
- video.rs: drop the unused GpuFrame::ten_bit field (present keys off `hdr`;
  the value is still computed locally for the first-frame log).
- present.rs: GpuView::frame is an RAII keep-alive (its Drop returns the decoder
  surface to the pool), never read -> #[allow(dead_code)].

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

clients/windows/src/gpu.rs

@@ -97,7 +97,9 @@ fn create_device() -> Result<SharedDevice> {
             // FFmpeg also sets this during hwdevice init, but doing it up front keeps the
             // cross-thread `Send`/`Sync` sound from the moment the device exists.
             if let Ok(mt) = context.cast::<ID3D11Multithread>() {
-                unsafe { mt.SetMultithreadProtected(true) };
+                unsafe {
+                    let _ = mt.SetMultithreadProtected(true); // returns the prior state; ignore
+                }
             }
             tracing::info!(
                 driver = if hardware {