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/service.rs

@@ -603,7 +603,8 @@ fn uninstall() -> Result<()> {
     Ok(())
 }
 
-/// Write a default `host.env` if none exists, so a fresh install streams with NVENC out of the box.
+/// Write a default `host.env` if none exists, so a fresh install streams out of the box. The encoder
+/// defaults to `auto` — the host picks NVENC (NVIDIA) / AMF (AMD) / QSV (Intel) from the GPU vendor.
 fn ensure_default_host_env() -> Result<()> {
     let path = host_env_path();
     if path.exists() {
@@ -616,7 +617,9 @@ fn ensure_default_host_env() -> Result<()> {
         # KEY=VALUE per line; '#' comments. Restart the service after editing:\n\
         #   punktfunk-host service stop && punktfunk-host service start\n\
         \n\
-        PUNKTFUNK_ENCODER=nvenc\n\
+        # Encode backend: auto (default) detects the GPU vendor — NVIDIA->nvenc, AMD->amf, Intel->qsv.\n\
+        # Force one with nvenc | amf | qsv | sw (software H.264). amf/qsv need an FFmpeg-built host.\n\
+        PUNKTFUNK_ENCODER=auto\n\
         PUNKTFUNK_VIDEO_SOURCE=virtual\n\
         PUNKTFUNK_SECURE_DDA=1\n\
         RUST_LOG=info\n\
@@ -625,7 +628,7 @@ fn ensure_default_host_env() -> Result<()> {
         # compat). Use `serve` for a SECURE native-only host (no GameStream #5/#9 surface).\n\
         # PUNKTFUNK_HOST_CMD=serve --gamestream\n\
         \n\
-        # Force a specific NVENC render GPU by name substring (multi-GPU boxes only):\n\
+        # Force a specific render GPU by name substring (multi-GPU boxes only):\n\
         # PUNKTFUNK_RENDER_ADAPTER=4090\n";
     std::fs::write(&path, default).with_context(|| format!("write {}", path.display()))?;
     println!("Wrote default config: {}", path.display());