cathar v0.7.7

VHS noise print no longer eats sibilance, the chain no longer gates pauses,
and a weak 50 Hz line is not mistaken for 60
(#27).

Fixed

• vhs noise print (#27) —
  learn_noise_print_quietest took one contiguous 4 s stretch. On dialogue
  that stretch sits well above the true pauses and the print carries
  sibilance, so subtraction takes down 8–16 kHz. The chain now stitches
  eight non-overlapping 0.75 s windows spread evenly by level over the
  quietest 20 % of the file, with 10 ms crossfades
  (learn_noise_print_quiet_windows). Digital-silent tiles are skipped so
  a dropout is not learned as the floor.
• vhs alpha (#27) —
  default over-subtraction is 2.0 (VhsOptions.alpha, cathar vhs --alpha).
  Alpha 3 with coherent subtraction gated pauses to near-silence.
• detect_mains_hz (#27) —
  50 vs 60 is taken from the mean spectrum of the whole recording, not the
  quietest 1 s. A weak 50 Hz PAL line was losing to a 60 Hz bump in one
  pause; harmonic placement still uses the quietest stretch.