The layered async step-down tests should run with precise_checkpoint=true so they exercise the checkpoint mode required by disaggregated storage.

Several tests intentionally stop while the step-down timestamp is armed. Under precise checkpointing, their stable constituent can retain dirty updates newer than stable, preventing teardown verification from closing the live handle. Complete the planned step-down after each test's final behavioral assertion so normal layered verification can inspect the durable checkpoint without changing the behavior under test. Keep tests that must remain leader-specific semantically intact with targeted stable/checkpoint cleanup.

Acceptance criteria:
* All active test_layered_async_stepdown tests enable precise_checkpoint.
* Tests complete or settle role state only after their behavioral assertions.
* Normal verifyLayered teardown remains enabled.
* The full layered_async_stepdown Python subsuite and dist/s_fast pass.