test: align conformance fixture surfaces

[rlippmann]

What changed

• Rename the shared multi-call fixture family from controller to workflow.
• Rename the historical follow-up case to describe the absence of continuation state.
• Update the Python conformance runner and fixture documentation for the new workflow family.
• Exercise grammar rendering through public CanonicalDirective construction and .text instead of the private renderer.
• Add retired apply_transcript to the forbidden Engine-member contract.

Why

• Keep the shared conformance contract aligned with the current 0.9 Engine API and avoid implying a retired controller surface.
• Ensure grammar rendering coverage uses the public grammar API without changing runtime behavior.
• Make historical API retirement explicit for ports.

Checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b7c9eb9) to head (7eb490b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #268   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          783       783           
=========================================
  Hits           783       783           

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