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/specs/frontend-mall/spec.md

@@ -25,12 +25,20 @@ A shopper SHALL be able to browse, view detail, add to cart, checkout with a shi
 - **THEN** the resulting order appears in the buyer center and the cart is empty
 
 ### Requirement: B2B2C mall-style PC storefront shell
-The mall SHALL render a buyer-facing desktop shell modeled on a classic B2B2C PC mall: a 30px utility bar, logo/search/cart header, dark primary navigation with a hover category mega-menu, a 1200px content grid, and a value-proposition footer. The visual language SHALL use #ca151e for brand/price/active states, #f5f5f5 section backgrounds, gray hairline borders, compact controls, and product-card hover lift/shadow. The implementation SHALL use Nuxt-native semantic components and SHALL NOT depend on Element Plus.
+The mall SHALL render a buyer-facing desktop shell modeled on a classic B2B2C PC mall: a 30px utility bar, logo/search/cart header, dark primary navigation with a hover category mega-menu, a 1200px content grid, and a value-proposition footer. The header SHALL remain fully visible while scrolling; no part of the shell SHALL auto-hide based on scroll position. The category mega-menu SHALL appear as a hover dropdown under the navigation "All Categories" entry on every page except the home page, where it is instead pinned in the hero row. The visual language SHALL use #ca151e for brand/price/active states, #f5f5f5 section backgrounds, gray hairline borders, compact controls, and product-card hover lift/shadow. The implementation SHALL use Nuxt-native semantic components and SHALL NOT depend on Element Plus.
 
 #### Scenario: shopper opens any mall page
 - **WHEN** a shopper navigates to a buyer-facing route
 - **THEN** the shared desktop shell wraps the route content and its nav/search/cart controls are usable
 
+#### Scenario: header persists while scrolling
+- **WHEN** a shopper scrolls any mall page
+- **THEN** the logo/search/cart header bar remains visible and is never collapsed or hidden by scroll position
+
+#### Scenario: hover categories on a non-home page
+- **WHEN** a shopper hovers "All Categories" in the navigation on a page other than `/`
+- **THEN** the category mega-menu dropdown appears below the navigation and hides again on mouse leave
+
 ### Requirement: Mock API adapter
 The mall SHALL default to a fixed-data mock adapter that implements the existing `@vmall/shared` API client surface. The adapter SHALL support auth, catalog, currency conversion, cart mutation, checkout, order/payment/shipment state, and invoices without a backend. A runtime configuration switch SHALL select the live API client later without changing page call sites.
 
@@ -39,11 +47,19 @@ The mall SHALL default to a fixed-data mock adapter that implements the existing
 - **THEN** browsing, cart, checkout, payment, orders and invoice pages return deterministic mock data and remain functional
 
 ### Requirement: Mock PC home page
-The mall home page SHALL render a hero carousel, a six-item quick-link strip with promotion tiles, and bilingual product floors with a left advert and product-card grid.
+The mall home page SHALL render a hero row composed of a pinned 240px category sidebar on the left and a hero carousel filling the remainder of the 1200px grid, both 450px tall and occupying layout space (not overlaid). The sidebar SHALL list all top-level categories with up to three child links each; hovering a top-level category SHALL expand the mega-menu panel to the right over the carousel. Banner images SHALL render at fixed 450px height, center-cropped horizontally to the narrower carousel width. Below the hero, the page SHALL render a six-item quick-link strip with promotion tiles and bilingual product floors with a left advert and product-card grid.
 
 #### Scenario: shopper lands on home
 - **WHEN** `/` loads in mock mode
-- **THEN** the carousel, quick links, promotions and every non-empty product floor render fixed mock content
+- **THEN** the category sidebar is visible to the left of the carousel without any hover or click, the carousel renders center-cropped banners at 450px height, and the quick links, promotions and every non-empty product floor render fixed mock content
+
+#### Scenario: sidebar stays while scrolling
+- **WHEN** a shopper scrolls the home page beyond 200px
+- **THEN** the category sidebar remains rendered in the hero row and does not auto-hide
+
+#### Scenario: expand a category
+- **WHEN** a shopper hovers a top-level category in the pinned sidebar
+- **THEN** the mega-menu panel expands to the right, overlaying the carousel with that category's child and grandchild links
 
 ### 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. It SHALL provide `/goods/[id]` with image gallery/zoom, bilingual name/subtitle, integer-minor-unit prices, attribute and SKU selection, stock-aware quantity, coupons/activity summary, store card, sales rail, and detail/comments/after-sale tabs.