feat(clients/windows): D3D11VA hardware decode in the session client — Vulkan chain becomes vulkan → d3d11va → software

The vendor-agnostic DXVA path for GPUs without Vulkan Video (Intel's Windows driver foremost,
which previously landed on CPU decode). Ported from the retired WinUI presenter's decoder with
its Intel-safe discipline intact (decode pool stays libavcodec-derived — a hand-built pool
broke Intel at the first SubmitDecoderBuffers), on a decode device LUID-matched to the
presenter's adapter.

Hand-off is a ring of shareable BGRA8 textures (SHARED_NTHANDLE | SHARED_KEYEDMUTEX) filled by
the fixed-function ID3D11VideoProcessor (NV12/P010 → BGRA8, colour spaces from the per-frame
CICP; PQ is tone-mapped to SDR by the processor — HDR-first boxes take Vulkan Video). BGRA is
deliberate: importing a multiplanar NV12 D3D11 texture device-losts on NVIDIA however it is
consumed (plane-view sampling and DMA copy both validation-clean, both TDR — bisected), while
single-plane RGBA D3D11↔Vulkan interop is the path Chromium/ANGLE exercise on every driver.
The presenter imports a slot's NT handle per frame (VK_KHR_external_memory_win32, gated on the
spec-required external-format probe) and blits it into the video image — no CSC pass; the DXGI
keyed mutex (key 0 both sides, drop-tolerant) is the cross-API lock and visibility barrier.

Verified live vs a real host at 5120x1440@240 HEVC on an RTX 4090: 240 fps, e2e 2.7/3.0 ms
p50/p95 under the Khronos validation layer — parity with Vulkan Video (2.6 ms); auto still
resolves vulkan on NVIDIA. PUNKTFUNK_DECODER=d3d11va forces it; import/present failures demote
to software on the existing streak contract.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-client-core/src/session.rs

@@ -333,6 +333,8 @@ fn pump(
                             #[cfg(target_os = "linux")]
                             DecodedImage::Dmabuf(_) => "vaapi",
                             DecodedImage::VkFrame(_) => "vulkan",
+                            #[cfg(windows)]
+                            DecodedImage::D3d11(_) => "d3d11va",
                         };
                         if total_frames == 1 {
                             let (w, h, path) = match &image {
@@ -340,6 +342,8 @@ fn pump(
                                 #[cfg(target_os = "linux")]
                                 DecodedImage::Dmabuf(d) => (d.width, d.height, "vaapi-dmabuf"),
                                 DecodedImage::VkFrame(v) => (v.width, v.height, "vulkan-video"),
+                                #[cfg(windows)]
+                                DecodedImage::D3d11(d) => (d.width, d.height, "d3d11va"),
                             };
                             tracing::info!(width = w, height = h, path, "first frame decoded");
                         }