docs(openspec): archive pin-home-category-menu

Move the completed change to
openspec/changes/archive/2026-09-17-pin-home-category-menu/ and merge its
delta into openspec/specs/frontend-mall/spec.md.

Modified requirements:
- B2B2C mall-style PC storefront shell: the header stays fully visible
  while scrolling (no scroll-based auto-hide), and the category mega-menu
  is a hover dropdown on every page except home, where it is pinned in
  the hero row. Adds the "header persists while scrolling" and "hover
  categories on a non-home page" scenarios.
- Mock PC home page: the hero row is a pinned 240px category sidebar plus
  a carousel filling the remainder, both 450px tall, with the mega-menu
  overlaying the carousel and banners center-cropped. Adds the "sidebar
  stays while scrolling" and "expand a category" scenarios.

openspec/changes/archive/2026-09-17-pin-home-category-menu/.openspec.yaml

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