feat(ui): 稳健跨 OS 衬线兜底栈 + 双主题 token 奇偶校验

- 决策：不自托管 CJK webfont（数 MB + 构建期外拉，违反 CSP 友好/首屏）
  改用跨 OS 系统衬线栈，保证中文标题可靠命中衬线，零 webfont 字节
- 新增 themeParity 测试：paper/night 两块 --color-* 必须成对，防漏配破面

apps/web/lib/ui/themeParity.test.ts

@@ -0,0 +1,42 @@
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+
+import { describe, expect, it } from "vitest";
+
+// 双主题 token 奇偶校验（P1-8）：paper 与 night 两块的 --color-* 变量必须成对，
+// 任一模式漏配某个颜色 token 都会让该主题回退为无效变量而破面——此测试提前拦截。
+
+const cssPath = fileURLToPath(
+  new URL("../../app/globals.css", import.meta.url),
+);
+const css = readFileSync(cssPath, "utf8");
+
+function colorVarsInBlock(selector: string): Set<string> {
+  const start = css.indexOf(selector);
+  if (start === -1) {
+    throw new Error(`未找到主题块选择器：${selector}`);
+  }
+  const open = css.indexOf("{", start);
+  const close = css.indexOf("}", open);
+  const body = css.slice(open + 1, close);
+  const names = body.match(/--color-[\w-]+/g) ?? [];
+  return new Set(names);
+}
+
+describe("theme token parity", () => {
+  const paper = colorVarsInBlock('[data-theme="paper"]');
+  const night = colorVarsInBlock('[data-theme="night"]');
+
+  it("defines at least the core palette in each theme", () => {
+    expect(paper.size).toBeGreaterThan(10);
+    expect(night.size).toBeGreaterThan(10);
+  });
+
+  it("keeps paper and night color tokens paired (no theme drops a --color-* var)", () => {
+    const missingInNight = [...paper].filter((name) => !night.has(name));
+    const missingInPaper = [...night].filter((name) => !paper.has(name));
+
+    expect(missingInNight, "night 缺失（paper 有）").toEqual([]);
+    expect(missingInPaper, "paper 缺失（night 有）").toEqual([]);
+  });
+});