fix(ls): preserve multiple directory headers

[roian6]

Summary

• Preserve per-directory headers while compacting rtk ls output for multiple directory operands.
• Keep empty directory operands attributed as dir: plus (empty) instead of collapsing all output into one unlabeled empty result.
• Add regression coverage for populated and empty multi-directory listings.

Fixes #2857.

Testing

• rustup run 1.91.0 cargo fmt --all -- --check
• rustup run 1.91.0 cargo clippy --all-targets
• rustup run 1.91.0 cargo test
• Manual smoke: rtk ls dir_a dir_b
• Manual smoke: rtk ls empty_a empty_b
• Manual smoke: rtk ls root.txt dir_a