docs: architecture, guidelines, domain designs, progressive code index

docs/code_index/index.md

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+# Code Index
+
+Progressive disclosure: pick the domain, open its file, get the concrete
+paths. **Update the affected domain file whenever you add/remove/rename
+files it lists.**
+
+| Domain | Covers | Index |
+|---|---|---|
+| Order & freight | checkout, orders, shipments, freight templates, shipping companies | [code_index/order.md](order.md) |
+| Post-order | aftersales/refunds, reviews | [code_index/post-order.md](post-order.md) |
+| Catalog | products, SKUs, categories, brands | [code_index/catalog.md](catalog.md) |
+| Marketing | coupons, flash sales, group buying, points | [code_index/marketing.md](marketing.md) |
+| Platform | identity, accounts/ledger, shops, storefront content, currencies, addresses | [code_index/platform.md](platform.md) |
+| Shared contract | types, API client, locales, UI components | [code_index/shared.md](shared.md) |
+| Storefront app | mall pages, mock adapter, live-domain wiring | [code_index/mall.md](mall.md) |
+| Console apps | shop-admin, admin pages | [code_index/consoles.md](consoles.md) |
+
+Cross-cutting references: `docs/architecture.md` (system),
+`docs/backend-guidelines.md`, `docs/frontend-guidelines.md`,
+`docs/development.md` (run/test), `openspec/specs/` (normative behavior).