Simplify grammar parsing API contract

[rlippmann]

What changed

• Removed redundant grammar API layers:
  • removed is_canonical_directive();
  • removed ValidatedDirective and validate_directive().
• Clarified the remaining grammar API responsibilities:
  • match_canonical_directive_start() for shallow syntax detection;
  • decompose_directive() for full directive parsing;
  • render_directive() for canonical directive serialization.
• Clarified directive text semantics:
  • CanonicalDirective.text and related text fields preserve accepted input text;
  • canonical serialized output is produced through render_directive().
• Strengthened grammar round-trip property coverage to verify rendered directives preserve both kind and operands.
• Removed duplicate grammar conformance fixture coverage.
• Bumped the development release version to 0.9.0dev7.

Why

• Reduce redundant public grammar surface area.
• Keep the grammar contract focused on parsing, detection, and rendering responsibilities.
• Make the Python/TypeScript grammar API easier to keep aligned.

Checklist

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