feat: CMS head and one JSON-LD graph on every server-rendered page (tracer)

- Nitro middleware resolves page requests once and hands the answer to the
  render (event.context.seo); the route middleware reuses it
- useCmsSeo: html lang, hreflang links, og:locale and exactly one JSON-LD
  script with one @graph (CMS nodes plus useSchemaNode nodes, merged by @id)
- local module (configKey seo): cms (alias of public.seo.enabled),
  breadcrumbs, links.newTabHint, rootFiles, resolveFiles
- GET /api/__seo__/resolve for client navigation through the Nuxt server
- smoke harness: count=, exact header=, noheader=, reqheader=, repeated
  options, resolves/apicalls/noreport lines, --no-site; v0.1.1 suite runs
  unchanged against a backend without /site

server/api/__seo__/resolve.get.ts

@@ -0,0 +1,24 @@
+// GET /api/__seo__/resolve?url=<path and query>: the resolve answer through
+// the layer's server (and its cache), for client-side navigation on a site
+// whose proxy does not route /_seo/api/v1 to the backend. The browser uses
+// it when public.seo.apiBase (NUXT_PUBLIC_SEO_API_BASE) is "/api/__seo__";
+// the default stays the backend's same-origin /_seo/api/v1.
+//
+// 404 when the layer is off; 422 for a url that is not a path; 502 when the
+// backend gives no usable answer. Never route-cached ('/api/__seo__/**'
+// cache false) and never indexed.
+
+import { seoServerConfig } from '../../lib/config.ts'
+import { resolveCached } from '../../lib/resolve.ts'
+
+export default defineEventHandler(async (event) => {
+  setResponseHeaders(event, { 'cache-control': 'no-store', 'x-robots-tag': 'noindex' })
+  if (!seoServerConfig(useRuntimeConfig(event)).enabled) throw createError({ statusCode: 404, statusMessage: 'Not Found' })
+  const url = getQuery(event).url
+  if (typeof url !== 'string' || !url.startsWith('/') || url.startsWith('//') || url.length > 2048) {
+    throw createError({ statusCode: 422, statusMessage: 'Bad url' })
+  }
+  const answer = await resolveCached(event, url)
+  if (!answer) throw createError({ statusCode: 502, statusMessage: 'No SEO answer' })
+  return answer
+})