- Store regular-token UPOS as plain strings instead of ClassLabel
  indices.
- Store regular-token HEAD as int16 instead of strings.
- Keep legacy ClassLabel UPOS reconstruction compatibility.
- Reject invalid or out-of-range regular-token HEAD values.
- Refresh helper documentation and release checklist.

Refs: https://github.com/bot-zen/ud-hf-parquet-tools/pull/1
Refs: https://huggingface.co/datasets/universal-dependencies/universal_dependencies/discussions/2