scan never records anything on a filesystem that is not persistent
(tmpfs, proc, sysfs and the other kernel or memory-backed types). As it
starts it notes the device ID of each such mount point, from
/proc/self/mountinfo on Linux and the mounted-filesystem list on macOS,
and checks every operand, directory and file the walk meets against
them; no flag turns this off. Below an operand the walk also skips the
default excludes, a path list per system (Linux, macOS, and Windows,
which is not used yet), and --no-default-excludes turns off only those.
A default exclude that is itself an operand gets no skip line. The tests
stop at once, saying why, when TMPDIR is not on a persistent filesystem.
Judgement call: an unreadable mount list fails the scan, and no flag gets round it.
Judgement call: a file on such a filesystem below an operand gets the same line as a directory.
Judgement call: an operand on such a filesystem that another operand's walk also reaches is named twice.
Unverified: the macOS mount list is built for macOS but never run; bind mounts need root to test.
Model: opus-5-5
