refactor: separate directive parsing and application

[rlippmann]

What changed

• Added Engine.apply_directive() as the semantic application boundary for parsed directives.
• Removed the legacy _parse_directive() path so directive handling now flows through:
  • decompose_directive()
  • CanonicalDirective
  • apply_directive()
• Removed leaked implementation types from the public API:
  • Action is now internal to the engine.
  • DirectiveKind is now internal to grammar.
• Updated tests, fixtures, and documentation to reflect the reduced public surface.

Why

The previous implementation mixed directive parsing and semantic application inside the engine.

This change separates responsibilities:

• grammar owns converting text into canonical directives;
• engine owns applying canonical directives to state.

Reducing the public surface prevents consumers from depending on internal semantic representations and keeps the supported integration path centered on parsed directives and engine application.

Checklist

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