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/specs/frontend-mall/spec.md

@@ -0,0 +1,18 @@
+# Spec Delta
+
+## MODIFIED Requirements
+
+### Requirement: Product discovery pages
+The mall SHALL provide `/search` with breadcrumb, category, brand and sort controls, a five-column desktop product grid, pagination and an empty state, listing products from the catalog API filtered by the selected category's subtree and brand. The sort control SHALL offer newest-first, price and sales. It SHALL provide `/goods/[id]` rendering product and SKU data from the catalog API with image gallery/zoom, bilingual name/subtitle, integer-minor-unit prices, attribute and SKU selection, stock-aware quantity, store card, and detail/after-sale tabs. Product cards and the product detail page SHALL show the product's real sold count, and the mall SHALL NOT present reviews, ratings or reviewer comments while no reviews capability exists.
+
+#### Scenario: filter and inspect a product
+- **WHEN** a shopper filters the search page by a parent category and a brand, then opens a product
+- **THEN** products from that category's subtree matching the brand are listed, and selecting an in-stock SKU updates the displayed price, stock and cart target from the catalog API
+
+#### Scenario: sort by sales
+- **WHEN** a shopper sorts the search results by sales
+- **THEN** the order follows the products' reported sold counts
+
+#### Scenario: no invented reviews
+- **WHEN** a shopper opens a product
+- **THEN** the page shows the shop's own after-sale copy and no rating, review count or reviewer comment