feat: add structured directive syntax failures

[rlippmann]

What changed

• Added structured failure details to InvalidDirectiveSyntax.
• Added public DirectiveKind and DirectiveSyntaxFailure classification types.
• Removed redundant engine-side syntax validation now owned by the grammar layer.
• Bumped the development version to 0.9.0dev9.

Why

• Give grammar consumers enough structured information to distinguish and explain invalid directive syntax without duplicating parser logic.
• Keep syntax validation in the shared grammar layer while leaving state-dependent semantic validation in the engine.
• Make the classification types exposed through public grammar results explicit parts of the public contract.

Checklist

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