feat: breadcrumbs component

- <SeoBreadcrumbs> (UI-SPEC L1): the resolve answer's trail as nav > ol
  with server-rendered links and the last item as aria-current text;
  nothing with fewer than two items, with page meta seo.breadcrumbs false
  or with the option seo.breadcrumbs false; props items, label,
  hideCurrent, locale
- one trail per page: the first rendered instance claims it
  (seoClaimBreadcrumbs, kept per path)
- seoSitePath: same-origin CMS URLs become paths, non-http(s) URLs are
  never links; useSeoLabels takes an optional locale
- L1 default styles with CSS-generated separators

testing/expect-seo.txt

@@ -84,3 +84,10 @@ noreport /stary
 # noindex route. The build matches the site answer: no drift warning.
 routereport 1 has=/ has=/plain has=/about lacks=/konto lacks=/szkic lacks=/prywatne
 log [vue-seo-layer] drift::0
+
+# L1 breadcrumbs: a layout instance (app.vue) and a page instance render
+# exactly one trail, server-rendered links, the last item as text with
+# aria-current; none on the home page (one item) or a page without a trail.
+200 /blog/lwia-grzywa count=<nav class="seo-breadcrumbs":1 contains=<a class="seo-breadcrumbs__link" href="/">Start</a> contains=<span class="seo-breadcrumbs__current" aria-current="page">Lwia grzywa</span> count=aria-current="page":1
+200 / count=seo-breadcrumbs:0
+200 /plain count=seo-breadcrumbs:0