Show topics on the home page, name the current one in the header

The home page fell back to PaperMod's recursive page list, which dumped
digest pages instead of the research topics they belong to. Add a home
layout that renders one card per entry in data/topics.yaml, with venue
counts, the nearest upcoming deadline, and section shortcuts.

With the topics listed there, drop the topic menu from the header and
show the current topic beside the site title instead, so the selected
research area is visible from every page. The breadcrumbs then repeated
that prefix, so override the partial to start below the topic section.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

site/layouts/partials/breadcrumbs.html

@@ -0,0 +1,20 @@
+{{- /* PaperMod's breadcrumbs start at Home and walk down every ancestor. The
+       header already shows "<site title> / <topic>", so the first two crumbs
+       would repeat it: drop the home page and the topic section, and render
+       nothing at all when that leaves the trail empty. */ -}}
+{{- if (.Param "ShowBreadCrumbs") -}}
+{{- $ancestors := .Ancestors.Reverse -}}
+{{- $crumbs := slice -}}
+{{- if gt (len $ancestors) 2 }}{{- $crumbs = after 2 $ancestors }}{{- end -}}
+{{- if $crumbs }}
+<nav class="breadcrumbs" role="navigation" aria-label="Breadcrumb">
+    {{- range $crumbs }}
+    <a href="{{ .RelPermalink }}">{{ .Title }}</a>
+    <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"
+        stroke-linecap="round" stroke-linejoin="round" class="feather feather-chevron-right">
+        <polyline points="9 18 15 12 9 6"></polyline>
+    </svg>
+    {{- end }}
+</nav>
+{{- end }}
+{{- end -}}