fix: reject premise directive compounds

[rlippmann]

What changed

• Reject recognized directive starters inside set premise and change premise to operands as COMPOUND_DIRECTIVE, including no-separator cases.
• Add shared grammar fixtures and Python/Hypothesis coverage for premise compound boundaries.
• Clarify the grammar specification and consolidate the shared compound-attempt rule for premise and policy operands.

Why

Premise operands must not absorb additional directive syntax. This keeps the 0.9 grammar contract consistent across implementations while preserving ASCII-only structural matching.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)
• strict grammar coverage remains at 100%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8341319) to head (07e961d).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #275   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          784       773   -11     
=========================================
- Hits           784       773   -11     

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