viser v0.5.0

Added

• viser-metrics crate — metric-vs-metric correlation (Pearson, Spearman/
  SROCC, Kendall/KROCC) and divergence detection across aligned score series.
• viser metrics compare — measure several encodes against one reference and
  compare the metrics: ranked per-metric table, best-per-metric, an agreement
  matrix, and CSV/JSON/HTML reports.
• viser metrics no-ref — pure-Rust no-reference signals (sharpness via
  variance of Laplacian, 8×8 blockiness, Immerkær noise) that need no reference.
• More metrics — MS-SSIM, VIF, CAMBI, and XPSNR, plus per-component (Y/U/V)
  PSNR and pooling strategies (harmonic mean, p1/p5/p10, median).

Changed

• Full-clip SSIMULACRA2/butteraugli by default — measured over every frame
  via a single-pass batch extract; --frame-samples N remains as a speed knob.

Fixed

• PSNR silently zero — libvmaf was sent repeated :feature= options, so PSNR
  was dropped whenever SSIM was also requested; features are now combined into a
  single |-separated option.