feat!: render the post page on the shared SEO layer with one H1 in both Journal H1 modes (tracer)

- extend vue-seo-layer (link dependency during development) and drop
  @nuxtjs/seo: the page head, canonical and the one JSON-LD graph come
  from the CMS resolve answer; the post's own title, excerpt and image
  apply only while there is no CMS answer
- remove the journal sitemap source and the copied shared purge route,
  library and plugin (the SEO layer serves POST /api/__site__/cache/purge);
  the deprecated POST /api/__journal__/cache/purge keeps working
- post title is h1 while the Journal API reports content_h1_to_h2 on
  (or absent) and p.journal-post__title while off; journal.postTitleTag
  ('auto' | 'h1' | 'p') overrides it; body h1 headings are demoted to h2
  while the setting is on (gateJournalHtml/sanitizeJournalHtml demoteH1)
- journal.breadcrumbs option and post-title CSS tokens
- smoke harness: count=, header=, noheader= and resolves lines; the
  fixture API also answers the SEO resolve and site endpoints

BREAKING CHANGE: the layer no longer installs @nuxtjs/seo (no
nuxt-site-config, nuxt-schema-org, robots or sitemap module, no
/api/__sitemap__/journal source, no useSiteConfig()); apps take their
head, sitemaps and robots from the SEO plugin through vue-seo-layer.

server/plugins/site-cache-purge.ts

@@ -1,48 +0,0 @@
-// Keeps the site cache purge settings out of the public site config.
-//
-// nuxt-site-config (pulled in by @nuxtjs/seo) copies every NUXT_SITE_* and
-// NUXT_PUBLIC_SITE_* environment variable into the site config, camel-cased,
-// and Nuxt serializes the site config into every page's payload (and into
-// window.__NUXT_SITE_CONFIG__ on client-only pages). The purge token's
-// earlier name NUXT_SITE_CACHE_PURGE_TOKEN therefore reached every page as
-// cachePurgeToken, and NUXT_SITE_CACHE_ALLOW_PROXIED as cacheAllowProxied.
-// The settings now live under NUXT_CACHE_PURGE_TOKEN and
-// NUXT_CACHE_PURGE_ALLOW_PROXIED, outside that prefix. The old names keep
-// working for one release, so this plugin also removes the two keys from
-// every entry of the site config stack on nuxt-site-config's
-// `site-config:init` hook, before anything reads it. The stripping stays
-// with the new names too: a site that still sets an old variable, or sets
-// both, must not publish it.
-//
-// When a setting comes from an old name, the plugin logs one deprecation
-// warning per process (globalThis[SITE_PURGE_WARNED], so a site extending
-// several layers that ship this file warns once). The warning names
-// variables, never a value.
-//
-// Without nuxt-site-config the hook is never called and the plugin does
-// nothing else. Shared: several layers ship this file byte for byte;
-// running it more than once is harmless.
-
-import { SITE_PURGE_WARNED, redactSiteConfigStack, resolveSitePurgeConfig, sitePurgeDeprecationMessage } from '../lib/siteCachePurge.ts'
-
-interface SiteConfigInitContext {
-  siteConfig?: { stack?: unknown }
-}
-
-type LooseHooks = { hook: (name: string, fn: (ctx: SiteConfigInitContext | undefined) => void) => unknown }
-
-export default defineNitroPlugin((nitroApp) => {
-  const g = globalThis as Record<symbol, unknown>
-  if (!g[SITE_PURGE_WARNED]) {
-    const message = sitePurgeDeprecationMessage(resolveSitePurgeConfig(useRuntimeConfig()).deprecated)
-    if (message) {
-      g[SITE_PURGE_WARNED] = true
-      console.warn(message)
-    }
-  }
-
-  // The hook name is untyped in an app without nuxt-site-config.
-  ;(nitroApp.hooks as unknown as LooseHooks).hook('site-config:init', (ctx) => {
-    redactSiteConfigStack(ctx?.siteConfig?.stack)
-  })
-})