Port section layouts (docs, paper, vault) and papers content

- layouts/docs/single.html: docs section layout with sidebar nav + TOC
- layouts/paper/single.html: essay layout with byline, abstract, tags
- layouts/vault/single.html: vault layout with sidebar nav + front-matter prev/next
- content/papers/on-semantic-html.md: 'On Semantic HTML as Agent Interface'
- content/papers/on-sessional-continuity.md: 'On Sessional Continuity'

All layouts use partial nav.html for dynamic menus. Paper layout
uses data-paper, data-abstract, data-byline, data-role=tag-cloud.
36 pages build clean, paper pages live at /papers/.

Closes: asw#14, asw#10

layouts/paper/single.html

@@ -0,0 +1,65 @@
+{{ define "main" }}
+  {{ partial "nav.html" . }}
+
+  <main>
+    <article data-paper>
+
+      <header>
+
+        <h1>{{ .Title }}</h1>
+
+        {{- with .Description -}}
+        <p data-abstract>{{ . }}</p>
+        {{- end -}}
+
+        {{/* Byline: date and/or author */}}
+        {{- $hasDate   := not .Date.IsZero -}}
+        {{- $hasAuthor := .Params.author -}}
+        {{- if or $hasDate $hasAuthor -}}
+        <p data-byline>
+          {{- if $hasDate -}}
+          <time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "January 2, 2006" }}</time>
+          {{- end -}}
+          {{- if and $hasDate $hasAuthor -}}
+          <span aria-hidden="true"> · </span>
+          {{- end -}}
+          {{- with .Params.author -}}
+          <span>{{ . }}</span>
+          {{- end -}}
+        </p>
+        {{- end -}}
+
+        {{/* Tags */}}
+        {{- with .GetTerms "tags" -}}
+        <nav data-role="tag-cloud" aria-label="Tags">
+          {{- range . -}}
+          <a href="{{ .Permalink }}" data-tag="{{ .Name }}">{{ .Name }}</a>
+          {{- end -}}
+        </nav>
+        {{- end -}}
+
+      </header>
+
+
+      {{ .Content }}
+
+
+      {{/* ── Prev / Next footer ──────────────────────────────────────── */}}
+      {{- if or .PrevInSection .NextInSection -}}
+      <footer>
+        {{- with .PrevInSection -}}
+        <a href="{{ .RelPermalink }}" rel="prev">← {{ .LinkTitle }}</a>
+        {{- end -}}
+        {{- with .NextInSection -}}
+        <a href="{{ .RelPermalink }}" rel="next">{{ .LinkTitle }} →</a>
+        {{- end -}}
+      </footer>
+      {{- end -}}
+
+    </article>
+  </main>
+
+  <footer>
+    <small><a href="/">{{ .Site.Title }}</a> · {{ .Date.Format "2006" }}</small>
+  </footer>
+{{ end }}