Add site search (Pagefind)

Client-side index generated after the Hugo build; search box in the
header, results page with a section filter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDRjtmAiNpo5mK5N3d7QwZ

layouts/history/single.html

@@ -1,10 +1,10 @@
 {{ define "main" }}
 <article>
-  <p class="crumbs"><a href="{{ .Parent.RelPermalink }}">History</a>{{ with .GetTerms "topics" }} › {{ partial "terms.html" (dict "page" $ "taxonomy" "topics") }}{{ end }}</p>
+  <p class="crumbs" data-pagefind-ignore><a href="{{ .Parent.RelPermalink }}">History</a>{{ with .GetTerms "topics" }} › {{ partial "terms.html" (dict "page" $ "taxonomy" "topics") }}{{ end }}</p>
   <h1>{{ .Title }}</h1>
   {{ with .Params.when }}<p class="meta">{{ . }}</p>{{ end }}
   <div class="content">{{ .Content }}</div>
-  <nav class="prev-next">
+  <nav class="prev-next" data-pagefind-ignore>
     {{ with .PrevInSection }}<a href="{{ .RelPermalink }}">← {{ .Title }}</a>{{ end }}
     {{ with .NextInSection }}<a href="{{ .RelPermalink }}">{{ .Title }} →</a>{{ end }}
   </nav>