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>

packaging/windows/punktfunk-host.iss

@@ -32,6 +32,11 @@
 #ifdef StageDir
   #define WithDriver
 #endif
+; FfmpegBin (a dir of FFmpeg shared DLLs) is optional — present when the host is built with
+; --features amf-qsv (the AMD/Intel AMF/QSV encode backend link-imports the FFmpeg libs).
+#ifdef FfmpegBin
+  #define WithFfmpeg
+#endif
 
 [Setup]
 AppId={{7C9E6A52-1F4B-4E8D-A3C7-2B5D8F1E0A93}
@@ -68,6 +73,12 @@ Name: "startservice"; Description: "Start the punktfunk host service now (also s
 Source: "{#BinDir}\punktfunk-host.exe"; DestDir: "{app}"; Flags: ignoreversion
 Source: "{#HostEnv}"; DestDir: "{app}"; Flags: ignoreversion
 Source: "{#Readme}"; DestDir: "{app}"; DestName: "README.txt"; Flags: ignoreversion
+#ifdef WithFfmpeg
+; FFmpeg shared DLLs (avcodec/avutil/swscale/...) laid down next to the exe — the AMD/Intel
+; (AMF/QSV) encode backend link-imports them, so the exe won't start without them. NVENC/software-
+; only builds simply omit this block.
+Source: "{#FfmpegBin}\*.dll"; DestDir: "{app}"; Flags: ignoreversion
+#endif
 #ifdef WithDriver
 ; The driver payload + nefconc.exe + install-sudovda.ps1, extracted to {tmp} and removed after install.
 Source: "{#StageDir}\*"; DestDir: "{tmp}\sudovda"; Flags: deleteafterinstall recursesubdirs createallsubdirs; Tasks: installdriver