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/present.rs

@@ -82,6 +82,8 @@ float4 ps_p010(VSOut i) : SV_Target {
 struct GpuView {
     y: ID3D11ShaderResourceView,
     c: ID3D11ShaderResourceView,
+    /// Held only for its `Drop` (returns the decoder surface to the reuse pool) — never read.
+    #[allow(dead_code)]
     frame: GpuFrame,
 }