refactor: expand public grammar API

[rlippmann]

What changed

• Promoted existing grammar directive classification helpers into the public grammar API.
• Removed redundant private-to-public delegation for grammar helpers and added public docstrings describing their behavior and boundaries.
• Updated the public grammar API fixture and grammar tests to cover the expanded export surface.

Why

• Grammar consumers need access to shared language classification behavior instead of maintaining duplicate implementations.
• The exported grammar API now better reflects the capabilities already owned by the grammar module.
• Documenting these functions clarifies that they perform grammar classification only and do not perform repair, inference, or state evaluation.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)