refactor: consolidate directive grammar API

[rlippmann]

What changed

• Made decompose_directive() the authoritative grammar parsing boundary.
• Added explicit invalid directive syntax classification:
  • CanonicalDirective
  • no directive (None)
  • InvalidDirectiveSyntax
• Reduced grammar API leakage:
  • made directive rendering internal;
  • made parser classification helpers internal.
• Consolidated grammar-defining literals and directive syntax constants.
• Preserved engine semantic behavior; this PR only changes grammar classification and ownership boundaries.

Why

The previous grammar API returned only CanonicalDirective | None, which required downstream consumers to reconstruct grammar distinctions.

This change makes grammar responsible for syntax classification while keeping semantic application in the engine layer.

Checklist

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