soundcraft-cli convert always passed dither: true, so a same-depth or
wider conversion still added ±1 LSB TPDF noise and digital silence came
out as noise. It now dithers only when writing integer samples that
cannot hold the source exactly: fewer bits than the source, a float or
lossy source, or samples changed by --rate/--normalize. 32-bit float
output never dithers. --dither and --no-dither override the choice,
like the Bounce option in #551.
