viser v0.4.2

Added

• FFmpeg version detection — check_ffmpeg() and check_ffprobe() validate
  that FFmpeg >= 6.0 is installed and report version info at startup (debug
  level). Catches missing or outdated binaries before any work begins.
• VMAF model validation — validate_vmaf_model() rejects unknown VMAF model
  names passed via --model, surfacing a clear error with the list of known
  models.
• STATUS.md — project roadmap and status tracker, linked from the README.

Fixed

• Banner image URL — crates.io now shows the viser banner (was a broken
  relative path).