Fault injection with the existing host fakes for FixedPatchStepper(
refinements=[]): an initialize upload failure, a BlockHaloCopy,
BlockCurvedBoundary or BlockStep launch failure, and a failing final
stream wait each raise, leave the instance failed (step/download refused)
and keep the completed step count; a failed launch never reaches the wait.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>