A workflow-level polymorphic association available on every Geneva Drive workflow. Links the workflow to any Active Record (payment, user account, email campaign, billing cycle). Most meaningful workflow state lives on the hero rather than inside the workflow; the workflow simply drives state transitions. Provides uniqueness via a DB constraint on (hero_type, hero_id, workflow_type) — e.g. only one billing_workflow per user. Deleting the hero cancels the workflow by default, but `continue_without_hero` is supported.