The LXC storage check runs in every mode, but its results were never
printed in the --dry-run summary, so a --dry-run appeared to skip LXC
storage entirely. Add an "LXC Storage:" section to the summary
(per-container usage %, a warning flag over the LXC_WARNING threshold, and
the issue count) via a small, testable helper `_format_lxc_dry_run()`.
Adds 4 regression tests.
Closes#23
Ref: #23
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>