test: align grammar parity coverage

[rlippmann]

What changed

• Enforced ASCII-only case-insensitive grammar matching for directive syntax.
• Added shared conformance coverage for full Unicode casefolding in decision messages.
• Added shared grammar coverage for opaque change premise values and newline-separated compound attempts.
• Added shared coverage proving Unicode casefold-equivalent inſtead of is not treated as the replacement delimiter.

Why

The current 0.9 grammar contract uses exact English literals with ASCII-only case-insensitive matching. These tests and the implementation keep Python behavior aligned with that portable contract while preserving Unicode case folding for policy identity and decision messages.

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 (01cac9c) to head (483a385).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #273   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          783       784    +1     
=========================================
+ Hits           783       784    +1     

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