feat(oakcodec): hardware video decoding by default on all platforms

FFmpeg 8 removed the standalone hardware decoders (h264_videotoolbox/
vaapi/nvdec/d3d11va no longer exist in its configure) — hardware decode
now only exists as a hwaccel attached to the software decoder. The new
oakcodec::hwdecode module therefore opens the regular decoder with the
platform's hardware device context attached (VideoToolbox on macOS,
VA-API then NVDEC on Linux, D3D11VA then NVDEC on Windows): FFmpeg
engages the matching hwaccel, decodes into hardware surfaces, and we
transfer them to system memory (NV12/P010) ahead of swscale.

- HardwareDecoding config switch, default ON by mandate; a checkbox in
  Preferences > Rendering (EN/ZH); device creation failure skips to the
  next candidate and finally to software; a decode-time failure on a
  hardware session reopens it as software and retries once.
- hw_decoder_name() observability hook plus a HW_TRANSFERS counter so
  tests can prove the hwaccel really engaged (not silently software).
- Verification: demo.mp4 H.264 decodes through VideoToolbox with a
  transferred hardware surface, and the pixels match the software
  decode within 0.05; switch off forces software.
- build-ffmpeg.sh also enables nvdec when ffnvcodec headers exist.

tooling/ffmpeg/build-ffmpeg.sh

@@ -182,10 +182,10 @@ case "$OS" in
 esac
 # NVIDIA (ffnvcodec headers are distribution-free; enable when present).
 if [ -d /usr/local/cuda ] || pkg-config --exists ffnvcodec 2>/dev/null; then
-	FLAGS+=(--enable-nvenc --enable-cuda-llvm)
-	echo "  + nvenc/cuda"
+	FLAGS+=(--enable-nvdec --enable-nvenc --enable-cuda-llvm)
+	echo "  + nvdec/nvenc/cuda"
 else
-	echo "  - nvenc/cuda (no ffnvcodec headers)"
+	echo "  - nvdec/nvenc/cuda (no ffnvcodec headers)"
 fi
 
 # --- Build ------------------------------------------------------------------