v0.3.0

Added

• concat convert command: merge Parquet files from a directory tree into a single file with optional profile_no / observation_no renumbering (on by default)
• concat header command: merge header YAML files into a single YAML file; errors on duplicate keys
• CI workflow: runs tests on push/PR to main (GitHub Actions)
• README.md

Changed

• concat is now a subcommand group (concat convert, concat header) for consistency with batch