Order Stages

A B2B order can be commercially active for weeks while nothing sits in a warehouse: confirmed, in production, quality control, ready for shipment. Today that lifecycle has no home. This gives merchants a parallel, per-store stage axis they compose visually in the dashboard: ordered stage definitions, a current-stage pointer on the order, per-stage customer visibility, an expected ready date, and events on every stage change. Stages are process tracking and presentation; semantic order statuses stay code-owned.

Plan: https://github.com/spree/spree/blob/main/docs/plans/6.1-order-stages.md

Tracked in Linear as V-3617.