scan now takes one or more PATH operands (directories or regular
files) via cobra flags instead of the -root flag with its /srv
default; invoking scan with no operand is a usage error and a
nonexistent operand is fatal. Filesystem boundaries are crossed by
default; the new -x/--one-file-system flag (GNU du/rsync convention)
stops the walk at each operand's filesystem, implemented by comparing
lstat device IDs with build-tagged helpers for darwin's int32 Dev.
Verified against a real mounted disk image: default crosses, -x does
not, --one-file-system is identical to -x.
Adds Description (name/purpose/category/license/author), Getting
Started, Rationale, TODO, License, and Author sections; the full
normative specification is preserved under Design. The stale non-goal
about having no git repository or CI is removed, and the Build section
now documents the Makefile targets.
