test: add engine property coverage

[rlippmann]

What changed

• added focused Hypothesis coverage for engine behavioral invariants in tests/test_properties.py
• covered step() vs apply_directive() equivalence for canonical directives
• added premise lifecycle state-machine properties for SET_PREMISE, CHANGE_PREMISE, CLEAR_PREMISE, and CLEAR_STATE
• added import_json() boundary properties for normalization convergence, rejection of empty normalized keys, and authoritative invariant preservation
• tightened a few existing property tests so they assert public behavior without overfitting parser-internal classification details
• this PR most directly strengthens coverage for Define identity normalization and case sensitivity rules #198 and partially supports Review public API mutation boundaries #201
• this PR also most directly reinforces PRs refactor: separate directive parsing and application #235 and refactor: complete 0.9.0dev8 cleanup #236, with partial support for refactor: consolidate directive grammar API #234 and feat: add structured directive syntax failures #237

Why

• engine normalization and state-transition rules are part of the user-facing contract and benefit from broader generated coverage than example-based tests alone
• the new properties raise confidence that equivalent entry points converge, invalid transitions do not mutate authoritative state, and imported state cannot bypass core invariants
• issue Define identity normalization and case sensitivity rules #198 tracks normalization and identity semantics, and these tests exercise the current contract in that area
• issue Review public API mutation boundaries #201 tracks mutation-boundary concerns, and these tests add coverage around authoritative state preservation across public engine operations
• recent parser and engine boundary refactors in refactor: consolidate directive grammar API #234, refactor: separate directive parsing and application #235, refactor: complete 0.9.0dev8 cleanup #236, and feat: add structured directive syntax failures #237 benefit from stronger generated regression coverage at those public boundaries

Checklist

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