What changed

• add Hypothesis coverage for state JSON invariants, deterministic replacement semantics, preview non-mutation, and grammar helper render/validate round trips in tests/test_properties.py
• refine generated strategies so the new properties stay within canonical supported inputs and deterministic replacement cases
• keep the PR test-only without changing runtime behavior

Why

• checkpoint removal simplified the authoritative core, so broader generated coverage helps catch state and transition edge cases that curated fixtures are unlikely to enumerate
• the new properties strengthen confidence in export/import stability, replacement determinism, preview isolation, and the public grammar helper contract

Checklist

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