The folder rail's tick state counts the selected tracks under each folder
by path prefix, `<folder>/…`. SACD tracks are selected under their virtual
path `sacd:<image>#N`, which never starts with the folder path, so a folder
holding ISOs showed an empty or partial tick while its tracks were
selected; the bar's count was right. Visible after a Shift range over a
folder of SACD images.
A folder now also counts `sacd:<folder>/…`, the image's tracks below an
ancestor, and `sacd:<folder>#…`, the tracks of the synthetic folder node
whose path is the image itself. Each form is one binary-searched range of
the sorted selection, so the per-publish cost stays the same.
Tests cover ancestors, the image node, mixed files and images, and a
sibling whose name extends the folder's.
cc
vicrodh
21bbcb126c