Documents the systemic non-hermetic-test hazard surfaced during ADR-0220: ~340 of
469 ChatRuntime constructions across 123 test files default to the shared
engine_state/ dir, so tests read/pollute each other's checkpoints (the spurious
test_achat identity-continuity warning). Records the interim rule (new/edited
tests must pass engine_state_path=tmp_path or no_load_state) and the recommended
future fix (root conftest autouse fixture monkeypatching engine_state._DEFAULT_DIR
+ CORE_ENGINE_STATE_DIR), deferred to its own full-suite-validated PR. Docs only.
