viser v0.6.0

Added

• Hardware encoder support — 10 hardware-accelerated codec variants:
  NVENC (h264_nvenc, hevc_nvenc), QSV (h264_qsv, hevc_qsv),
  VideoToolbox (h264_videotoolbox, hevc_videotoolbox),
  VAAPI (h264_vaapi, hevc_vaapi), AMF (h264_amf, hevc_amf).
• Runtime HW encoder detection via ffmpeg -encoders — available hardware
  encoders are discovered at startup and surfaced automatically.
• Per-backend rate-control dispatch — CRF, QP, and VBR modes mapped to
  each encoder's native syntax (e.g. NVENC -cq, QSV -global_quality).
• Per-backend preset mapping — NVENC p1–p7, QSV speed levels,
  VAAPI compression levels, AMF quality/speed, VideoToolbox real-time flag.