Add news posts with banner summaries on the home page

Posts live in content/post/; the five latest are summarised on the home
page. Builds for check/deploy go to build/ so they do not collide with a
running hugo server writing to public/.

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

layouts/post/single.html

@@ -0,0 +1,13 @@
+{{ define "main" }}
+<article class="post">
+  <p class="crumbs"><a href="{{ .Parent.RelPermalink }}">{{ .Parent.Title }}</a></p>
+  <h1>{{ .Title }}</h1>
+  <p class="meta">{{ if not .Date.IsZero }}<time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "2 January 2006" }}</time>{{ end }}{{ with .Params.author }}{{ if not $.Date.IsZero }} · {{ end }}{{ . }}{{ end }}</p>
+  {{ with partial "banner.html" (dict "page" . "width" 1000) }}<img class="post-banner" src="{{ . }}" alt="">{{ end }}
+  <div class="content">{{ .Content }}</div>
+  <nav class="prev-next">
+    {{ with .PrevInSection }}<a href="{{ .RelPermalink }}">← {{ .Title }}</a>{{ end }}
+    {{ with .NextInSection }}<a href="{{ .RelPermalink }}">{{ .Title }} →</a>{{ end }}
+  </nav>
+</article>
+{{ end }}