refactor: complete 0.9.0dev8 cleanup

[rlippmann]

What changed

• removed the internal _Action compatibility layer and routed directive handling directly through CanonicalDirective
• added focused Engine.apply_directive(...) boundary coverage for all directive kinds and representative semantic error cases
• removed create_engine() from the public Python API and updated local callers, docs, fixtures, demos, examples, and tests to use Engine() directly
• bumped the dev version to 0.9.0dev8

Why

• the grammar/application boundary refactor made the _Action shim unnecessary, so this cleans up the engine internals and keeps tests aligned with the live canonical-directive boundary
• create_engine() did not provide behavior beyond Engine(), so removing it simplifies the public API and keeps the package surface aligned with the implementation

Checklist

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