chore(openspec): archive wave 6 and close the migration tracker

The merge creates the brand capability (two requirements) and extends the
catalog browse requirement with the brand filter, the sales sort and a real
sold count. openspec validate --all --strict stays green at 14 specs.

With every box now checked, docs/TBD-migrate-wave.md is deleted, as its own
instructions require. Its two durable notes move to the README instead of
disappearing with it:

- `nuxt build` does not typecheck, so a green build is not the type gate the
  verification section claimed. AGENTS.md told me to treat it as one, which is
  why waves 1-5 over-reported "builds pass" as evidence.
- the list of what stays deliberately mock: addresses, coupons, favourites,
  account stats, the seckill/collective/integral pages, reviews, and the
  fixed-data adapter itself, which the adapter spec requires for rollback.

The README's verification block also gains the real test count (29, not 16).

openspec/specs/brand/spec.md

@@ -0,0 +1,28 @@
+# brand Specification
+
+## Purpose
+The product brand registry: a bilingual, admin-managed list of manufacturers that products reference, shoppers filter listings by, and the storefront shows on a product.
+
+## Requirements
+
+### Requirement: Public brand list
+`GET /api/brands` SHALL return the brands without authentication, ordered by their stored position, each with an id, slug and bilingual name. Products SHALL carry a nullable `brand_id` referring to that list, and `GET /api/products` SHALL accept a `brand_id` filter that composes with the existing category, shop and keyword filters.
+
+#### Scenario: filter a listing by brand
+- **WHEN** a shopper filters the catalog by a brand
+- **THEN** only that brand's published products are returned, and the filter combines with a category filter rather than replacing it
+
+#### Scenario: a product without a brand
+- **WHEN** a product has no brand assigned
+- **THEN** it is still listed and its `brand_id` is null rather than pointing at an invented brand
+
+### Requirement: Brand management
+A platform admin SHALL replace the brand list with `PUT /api/admin/brands`, which validates each entry, applies as one transaction and reindexes positions from the submitted order; the write SHALL require the `platform_admin` role. Each entry SHALL carry a slug and non-empty `en` and `zh` names, and a rejected list SHALL leave the stored brands untouched.
+
+#### Scenario: replace round-trips
+- **WHEN** an admin replaces the brands and then reads them publicly
+- **THEN** the public list matches, in the submitted order
+
+#### Scenario: incomplete bilingual name is refused
+- **WHEN** an admin submits a brand with only `en` text
+- **THEN** the request fails and the stored brands are unchanged