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/machine-tabs/single.html

@@ -3,7 +3,7 @@
 {{ $m := site.GetPage .Params.machine }}
 {{ $tab := .Params.tab }}
 {{ $p := $m.Params }}
-<article class="machine">
+<article class="machine" data-pagefind-meta="title:{{ .Title }} – {{ .Params.tab | title }}">
   {{ partial "machine-header.html" (dict "machine" $m "tab" $tab) }}
 
   {{ if in (slice "photos" "adverts" "hardware" "connectors") $tab }}