chore(openspec): archive add-group-buying

Sync the group-buying capability plus the order, frontend-mall, and
frontend-shop-admin requirements into the main specs, and write the capability
Purpose the archive leaves as a placeholder. This closes the last of the five
planned marketing changes.

openspec/changes/archive/2026-09-18-add-group-buying/specs/frontend-mall/spec.md

@@ -0,0 +1,16 @@
+## ADDED Requirements
+
+### Requirement: Live group-buying page
+The mall SHALL render active group-buying activities and open groups from the shared API client, including group price, required member count, current paid-member count, and expiry. It SHALL let an authenticated shopper choose to open or join a group and replace derived fixture counts. Checkout SHALL submit the open-or-join intent for a single activity SKU at quantity 1. Payment and order-detail views SHALL show the snapshotted group price. This change SHALL NOT add group price or join CTAs to the catalog product-detail page (`/goods/[id]`); opening and joining happen from the group-buying page and checkout intent.
+
+#### Scenario: open a group
+- **WHEN** a shopper selects an active group-buying product and chooses to open a group
+- **THEN** the checkout intent identifies the activity and requests a new group
+
+#### Scenario: order shows group snapshot
+- **WHEN** checkout succeeds with group-buy intent
+- **THEN** payment and order-detail views show the server group unit price from the order payload
+
+#### Scenario: product detail stays on catalog price
+- **WHEN** a shopper opens a catalog product that also has an active group-buying activity
+- **THEN** the product-detail page still shows catalog SKU prices and does not require a group-buy overlay in this change