docs(openspec): plan wave 6, brands and real sales

Planning only: proposal, delta specs, design and tasks. No code yet.

This is the last substantive in-scope box in docs/TBD-migrate-wave.md. Wave 1
removed the brand facet and the sales/comments sorts for want of a model; sales
are derivable from order_items and a brand model is a table plus a column, so
both come back for real.

Decisions recorded in the design:

- brands are reference data in their own table with a nullable products.brand_id
  and an ordered replace endpoint, mirroring categories and storefront content
- a "sale" is a unit on an order that reached payment; pending and cancelled
  orders do not count, so an abandoned checkout cannot inflate the figure
- sold_count is computed per read rather than stored, so it cannot drift from
  the orders that produced it
- the review UI is removed rather than relabelled: the mall attributes invented
  comments to named shoppers and shows a "good rate", which a migration that
  makes everything else real has no business keeping on screen. Reviews are
  recorded as a separate future capability

openspec validate --strict passes and the change is ready to apply.

openspec/changes/replace-mock-api-wave-6/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-09-17