test: extend portable conformance contracts

[rlippmann]

What changed

• Add shared read-only property metadata for engine.premise and engine.policies.
• Add shared constructor-input isolation coverage for CanonicalDirective.
• Enforce both contracts in the Python conformance harness while retaining Python-specific assignment tests.

Why

• Make these portable 0.9 guarantees explicit for future language 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 (c631f9f) to head (ccfd024).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #269   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          783       783           
=========================================
  Hits           783       783           

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