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>

packages/utils/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "@yuxingu/utils",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts"
+  }
+}

packages/utils/src/index.ts

@@ -0,0 +1,20 @@
+/** Clamp a number into [min, max]. */
+export function clamp(n: number, min: number, max: number): number {
+  return Math.min(max, Math.max(min, n))
+}
+
+/** Pad date part to 2 digits. */
+export function pad2(n: number): string {
+  return String(n).padStart(2, '0')
+}
+
+/**
+ * Validate calendar-ish birth inputs.
+ * Side effect: none. Returns error message or null if ok.
+ */
+export function validateBirth(year: number, month: number, day: number): string | null {
+  if (!Number.isFinite(year) || year < 1900 || year > 2100) return '年份无效'
+  if (month < 1 || month > 12) return '月份无效'
+  if (day < 1 || day > 31) return '日期无效'
+  return null
+}