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 -}}