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/Cargo.toml

@@ -184,6 +184,12 @@ vigem-client = { version = "0.1", features = ["unstable_xtarget_notification"] }
 # the crate builds on docs.rs/CI. We enable it so the GPU-less VM/CI compiles; the DirectX NVENC path
 # never calls CUDA at runtime, so the pinned CUDA bindings version is irrelevant.
 nvidia-video-codec-sdk = { version = "0.4", features = ["ci-check"], optional = true }
+# AMD (AMF) + Intel (QSV) hardware encode on Windows via libavcodec — the analogue of the Linux
+# VAAPI backend (`src/encode/ffmpeg_win.rs`). Optional + behind the `amf-qsv` feature because it
+# link-imports the FFmpeg libs at build time (needs a `FFMPEG_DIR` with the AMF/QSV encoders — the
+# same BtbN gpl-shared tree the Windows client uses) and pulls the shared `avcodec/avutil/...` DLLs
+# at runtime. `ffmpeg-sys-next` auto-detects the FFmpeg version (7.x/avcodec-61 or 8.x/62).
+ffmpeg-next = { version = "8", optional = true }
 
 [features]
 # NVENC hardware encode (Windows). OFF by default: it pulls the NVENC SDK, and the host then needs
@@ -191,3 +197,7 @@ nvidia-video-codec-sdk = { version = "0.4", features = ["ci-check"], optional =
 # time — i.e. `nvencodeapi.lib` from the NVIDIA Video Codec SDK (or an import lib generated from
 # nvEncodeAPI64.dll) on the linker path. Build the GPU host with `--features nvenc`.
 nvenc = ["dep:nvidia-video-codec-sdk"]
+# AMD/Intel hardware encode on Windows (AMF/QSV via ffmpeg-next). OFF by default: it needs a
+# `FFMPEG_DIR` (BtbN gpl-shared, includes `*_amf`/`*_qsv`) at build time and bundles the FFmpeg
+# DLLs at runtime. Build the all-vendor GPU host with `--features nvenc,amf-qsv`.
+amf-qsv = ["dep:ffmpeg-next"]