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/_partials/banner.html

@@ -0,0 +1,14 @@
+{{/* dict "page" <post> "width" <px> -> URL of the post banner, resized when it is a bundle image */}}
+{{- $url := "" -}}
+{{- with .page.Params.banner -}}
+  {{- with $.page.Resources.GetMatch . -}}
+    {{- if and (eq .MediaType.MainType "image") (ne .MediaType.SubType "svg") -}}
+      {{- $url = (.Resize (printf "%dx" $.width)).RelPermalink -}}
+    {{- else -}}
+      {{- $url = .RelPermalink -}}
+    {{- end -}}
+  {{- else -}}
+    {{- $url = . -}}
+  {{- end -}}
+{{- end -}}
+{{- return $url -}}

layouts/_partials/post-summary.html

@@ -0,0 +1,11 @@
+<article class="post-summary">
+  {{ with partial "banner.html" (dict "page" . "width" 640) }}
+  <a class="banner" href="{{ $.RelPermalink }}"><img src="{{ . }}" alt="" loading="lazy"></a>
+  {{ end }}
+  <div>
+    <h3><a href="{{ .RelPermalink }}">{{ .Title }}</a></h3>
+    <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>
+    <div class="summary">{{ .Summary }}</div>
+    {{ if .Truncated }}<p><a href="{{ .RelPermalink }}">Read more →</a></p>{{ end }}
+  </div>
+</article>