refactor: standardize all Hugo templates — consistent landmark structure

All templates now follow: body > nav > article > header + content + footer

Removed:
- <hgroup> (redundant, header+h1+p serves same purpose)
- <main> (article is the content landmark)
- role="main" on article (incorrect ARIA)
- header block in baseof (header now inside article)

Standardized: default, docs, essay, notes, console, page, list, index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

site/layouts/_default/single.html

@@ -1,23 +1,39 @@
-{{ define "header" }}
-  <header>
-    {{ with .Type }}<p data-text="eyebrow">{{ . }}</p>{{ end }}
-    <p data-text="dim">
-      <time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "January 2006" }}</time>
-      {{ with .Params.author }} · {{ . }}{{ end }}
-    </p>
-    {{ with .Params.tags }}
-    <p>{{ range . }}<a href="/tags/{{ . }}" data-text="tag">#{{ . }}</a> {{ end }}</p>
-    {{ end }}
-  </header>
-{{ end }}
-
 {{ define "content" }}
-  <article role="main">
+  <article>
+    <header>
+      <h1>{{ .Title }}</h1>
+      {{- with .Description }}
+      <p>{{ . }}</p>
+      {{- end }}
+      {{- if or (not .Date.IsZero) .Params.author }}
+      <p data-text="dim">
+        {{- if not .Date.IsZero -}}
+        <time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "January 2, 2006" }}</time>
+        {{- end -}}
+        {{- if and (not .Date.IsZero) .Params.author }} · {{ end -}}
+        {{- with .Params.author }}{{ . }}{{ end -}}
+      </p>
+      {{- end }}
+      {{- with .GetTerms "tags" }}
+      <nav aria-label="Tags">
+        {{- range . }}
+        <a href="{{ .Permalink }}">#{{ .Name }}</a>
+        {{- end }}
+      </nav>
+      {{- end }}
+    </header>
+
     {{ .Content }}
-    {{ with .Params.footer }}
+
+    {{- if or .PrevInSection .NextInSection }}
     <footer>
-      <small>{{ . }}</small>
+      {{- with .PrevInSection }}
+      <a href="{{ .RelPermalink }}" rel="prev"><span aria-hidden="true">←</span> {{ .LinkTitle }}</a>
+      {{- end }}
+      {{- with .NextInSection }}
+      <a href="{{ .RelPermalink }}" rel="next">{{ .LinkTitle }} <span aria-hidden="true">→</span></a>
+      {{- end }}
     </footer>
-    {{ end }}
+    {{- end }}
   </article>
 {{ end }}