V11: Episode timestamp precision and workflow ordering.

• Promotes started_at and finished_at from second-precision (:utc_datetime) to microsecond-precision (:utc_datetime_usec) so that sequential episodes naturally sort correctly without tiebreakers.
• Promotes queued_at to :utc_datetime_usec for the same reason.
• Adds workflow_step_no — the topological depth of the episode's step in the workflow DAG (root steps = 0, each dependent layer +1). Used as a stable tiebreaker for truly parallel steps that fire at the same instant.