LBMStepper.step raises before any launch or buffer swap when the owner has
set 'blocked'; Simulation sets it when a translating circle is declared, at
initialize and after stepper recreation. Tests: direct-stepper guard (zero
launches, no swap, unchanged step count), spin/multi-step/checkpoint run
variants, run(0) keeps the pending target, static runs proceed, mixed
moving/static packing parity with the moving id absent, failure refusal of
checkpoint calls, no retry and no re-entry after close.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
