feat(windows): AMD (AMF) + Intel (QSV) hardware encode on the Windows host

The Windows host was NVIDIA-only (NVENC) with an openh264 software fallback. Add
AMD AMF and Intel QSV via libavcodec — the Windows analogue of the Linux VAAPI
backend — so one installer serves all three GPU vendors.

- encode/ffmpeg_win.rs: new WinVendor{Amf,Qsv} encoder. System-memory NV12/P010
  readback (default, robust) + opt-in zero-copy D3D11 (PUNKTFUNK_ZEROCOPY: shares
  the capturer's ID3D11Device; AMF takes AV_PIX_FMT_D3D11, QSV derives a QSV frames
  ctx and maps) with a system fallback for the format-group mismatch the capturer's
  video-processor fallback can produce. HDR Main10 (P010 + BT.2020/PQ VUI; an
  Rgb10a2->P010 swscale covers the shader fallback).
- encode.rs: Codec::amf_name/qsv_name; open_video + windows_resolved_backend()
  resolve PUNKTFUNK_ENCODER=auto|nvenc|amf|qsv|sw via a DXGI adapter VendorId probe.
- capture/dxgi.rs: gpu_mode mirrors the resolved backend (D3D11 NV12/P010 for AMF/QSV).
- gamestream/serverinfo.rs: GPU-aware codec advertisement (windows_codec_support;
  AV1 gated to RDNA3+/Arc, like the VAAPI path).
- Cargo.toml: amf-qsv feature (optional ffmpeg-next in the windows target block).
- CI/installer: windows-host.yml sets FFMPEG_DIR + builds --features nvenc,amf-qsv;
  the Inno installer bundles the FFmpeg DLLs; host.env default nvenc -> auto.

CI-green target; AMF/QSV not yet on-glass validated (no AMD/Intel Windows box in the
lab) — NVENC stays live-validated. An adversarial-review pass caught + fixed real
FFI bugs (AV_PIX_FMT_P010 is a macro -> P010LE; windows-rs 0.62 GetImmediateContext/
GetDesc1 return Result; AV_HWFRAME_MAP_* is a bindgen enum with no BitOr).

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

crates/punktfunk-host/src/capture/dxgi.rs

@@ -2157,9 +2157,14 @@ impl DuplCapturer {
                 .ok()
                 .and_then(|s| s.parse().ok())
                 .unwrap_or((2000 / refresh_hz.max(1)).max(100));
-            let gpu_mode = std::env::var("PUNKTFUNK_ENCODER")
-                .map(|v| matches!(v.to_ascii_lowercase().as_str(), "nvenc" | "hw" | "nvidia"))
-                .unwrap_or(false);
+            // Produce GPU-resident D3D11 frames (zero-copy NVENC, or the NV12/P010 the AMF/QSV
+            // backends read back / import) whenever the resolved encode backend is a GPU one — so the
+            // capturer's output format matches the encoder's input. Only the software (GPU-less) path
+            // takes CPU staging. Mirrors `encode::open_video`'s dispatch exactly.
+            let gpu_mode = !matches!(
+                crate::encode::windows_resolved_backend(),
+                crate::encode::WindowsBackend::Software
+            );
             // Read the source display's HDR mastering metadata while we still hold `output` (it is
             // moved into the struct below). Only meaningful for an HDR (FP16) duplication.
             let is_hdr_init = dd.ModeDesc.Format == DXGI_FORMAT_R16G16B16A16_FLOAT;