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>

scripts/windows/host.env.example

@@ -9,9 +9,11 @@
 # Format: KEY=VALUE per line; '#' starts a comment. The service loads these into its environment
 # and passes PUNKTFUNK_* and RUST_LOG through to the host it launches into the active session.
 
-# Hardware encode via NVENC (NVIDIA). The host must be the `--features nvenc` build. Falls back to
-# the software encoder automatically if NVENC is unavailable.
-PUNKTFUNK_ENCODER=nvenc
+# Hardware encode backend. `auto` (default) detects the GPU vendor: NVIDIA->nvenc (direct SDK),
+# AMD->amf, Intel->qsv (both libavcodec). Force one with: nvenc | amf | qsv | sw (software H.264).
+# nvenc needs the `--features nvenc` build; amf/qsv need the `--features amf-qsv` build (FFmpeg DLLs
+# ship in the installer). The published installer is built with all three.
+PUNKTFUNK_ENCODER=auto
 
 # Video source: `virtual` creates a per-client virtual display (SudoVDA) at the client's exact
 # resolution + refresh — the flagship mode. Requires the SudoVDA indirect display driver installed.