v0.3.0

Adds --depth as a generalization of the no-recursion case other
duplicate finders spell as the absence of -r. --depth 0 limits the
scan to files directly in the named directories; --depth N allows N
levels of subdirectory descent; omitted, the scan remains unbounded
as before.

Also includes an internal refactor: the binary-only modules moved
from src/bin_support/ to src/cli/, and the inner clap-definitions
file was renamed from cli.rs to args.rs to keep import paths flat.
No user-facing impact from the rename.

See CHANGELOG.md for the full entry.