chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

.ai/design/platform/h5.md

@@ -0,0 +1,47 @@
+# Platform Contract — H5 (Mobile Web)
+
+Primary shipping client: `apps/user-h5` (Vue 3 + Vite).  
+Design language: [../design-system.md](../design-system.md) · Components: [../component-catalog.md](../component-catalog.md)
+
+---
+
+## MUST
+
+- Mobile-first; content column `max-width: var(--layout-max-user)` (430px), centered on large screens.
+- Touch targets ≥ 44×44px.
+- Prefer bottom actions (TabBar, AskInputBar, sticky CTA).
+- Safe area: `env(safe-area-inset-bottom)` on TabBar / fixed footers.
+- `user-scalable` / viewport per existing app shell; no desktop-only hover for essentials.
+- Import tokens from `@yuxingu/ui` / `packages/ui` — no page-local brand hex.
+- Data via `src/api` → `@yuxingu/sdk` only (no raw `fetch` in pages).
+- Structure: `pages/` · `components/` · `layouts/` · `stores/` · `router/`.
+- Loading / empty / error on every network view.
+- Report / Decode / Ask: compliance disclaimer.
+
+---
+
+## MUST NOT
+
+- Rely on hover tooltips for critical info.
+- Use `window` APIs that break iOS WeChat/browser without fallback.
+- Invent a second tab IA; tabs = Home / Explore / Ask / Companion / Mine.
+- Grow Legacy root HTML (`yuxingu.html`, `pages/`) for new UI.
+
+---
+
+## Layout notes
+
+- HomeWash: peach gradient body + Decode form + white ContentSheet.
+- Secondary: TopBar + padded content.
+- Keyboard: inputs must not hide primary CTA (scroll / visualViewport aware when needed).
+
+---
+
+## Checklist before Done
+
+- [ ] Tokens only
+- [ ] Catalog component reused or catalog updated
+- [ ] 44px touch
+- [ ] Safe area
+- [ ] States + compliance if needed
+- [ ] `npm run build:h5`