fix: preserve comment-only KDL blocks

[tomatotomata]

Summary

• preserve whether a parsed node had a block, even when the block contains only comments
• use that block marker when serializing so comment-only blocks keep their braces
• add a round-trip regression test for a comment-only block followed by another node

Closes #43.

Validation

• python -m unittest discover -s tests -p 'test_kdl_parser.py' -v: 28 passed; 5 existing Windows mode-bit failures in config-write tests (0o600/0o640/0o6750 assertions)
• focused comment-only block reproduction passes
• python -m ruff format --check nirimod/kdl_parser.py tests/test_kdl_parser.py passes
• git diff --check passes
• targeted Ruff reports only three pre-existing E501 findings in unchanged lines

The change is limited to the parser state and its regression coverage. Let me know what you think.

[srinivasr]

@tomatotomata
fix looks good. pulled it locally and tests pass, merging