Model-property-unit test hierarchy

Prefer model-based tests for stateful systems, property-based tests for parsers, validation, round trips, symmetry, and idempotence, then unit tests for trivial or hard-to-model cases. Chose invariant-focused tests over example-only coverage because examples miss combinations and transitions. When property tests replace existing unit cases, remove redundant tests and add a unit-file header naming the invariants covered by the property-test file.