Upgrading Spree 5.x → 6.0

Spree 6.0 is the breaking-change release. Cart and Order are separate models, state machines are gone (statuses + workflows), adjustments became typed rows (TaxLine / Discount / Fee), the master variant is gone, shipping became fulfillment/delivery, taxons became categories/collections, users became Spree::Customer, staff permissions became data, and the Rails admin, Rails storefront and API v2 were removed in favor of the React dashboard (spree_dashboard) and API v3.

The upgrade is: preconditions → gems → migrations → backfills → code fixes → behavioral review. Budget real time for the last two; the rake part is the easy part.

Generic mechanics (spree upgrade flags, STEP=, production release phase) are in the spree-upgrade skill. This skill is the 6.0-specific playbook.

1. Preconditions — do these before touching the Gemfile