feat(seo): add sitemap, canonical metadata and legacy redirects

- Split the config into a createConfig(target) factory plus a thin .cn
  wrapper, so the two deployments differ in one place. The .cn build
  points canonical, hreflang, og:url, sitemap and robots at that origin,
  and omits social links that are not reachable from mainland China.
- Generate sitemap.xml and robots.txt per target. robots.txt is written
  at build time because one static file cannot carry the right origin for
  both domains.
- Emit per-page canonical, hreflang and Open Graph tags. og:title and
  friends are left to the theme, which already emits them and does not
  de-duplicate against site-level head entries.
- Redirect the retired /projects/ paths on Vercel, before the CN redirect
  so those visitors get a single hop.

rspress.cn.config.ts

@@ -0,0 +1,10 @@
+/**
+ * Build configuration for `onixbyte.cn`.
+ *
+ * Thin wrapper over the shared factory — see `rspress.config.ts` for what the
+ * `cn` target changes. Invoke it with `pnpm build:cn`, which passes this file
+ * via `rspress build --config`.
+ */
+import { createConfig } from "./rspress.config"
+
+export default createConfig("cn")