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/docs/single.html

@@ -0,0 +1,113 @@
+{{ define "main" }}
+  {{ partial "nav.html" . }}
+
+  <main>
+    <div data-layout="docs">
+
+      {{/* ── Left sidebar ─────────────────────────────────────────────── */}}
+
+      <aside>
+        <nav aria-label="Documentation" data-nav="sidebar">
+
+          {{- $menu := index .Site.Menus "docs" -}}
+          {{- if $menu -}}
+
+            {{- range $menu -}}
+              {{- if .HasChildren -}}
+              <small>{{ .Name }}</small>
+              <ul>
+                {{- range .Children -}}
+                <li>
+                  <a href="{{ .URL }}"
+                    {{- if eq (relURL .URL) $.RelPermalink }} aria-current="page"{{ end -}}>
+                    {{- .Name -}}
+                  </a>
+                </li>
+                {{- end -}}
+              </ul>
+              {{- else -}}
+              <ul>
+                <li>
+                  <a href="{{ .URL }}"
+                    {{- if eq (relURL .URL) $.RelPermalink }} aria-current="page"{{ end -}}>
+                    {{- .Name -}}
+                  </a>
+                </li>
+              </ul>
+              {{- end -}}
+            {{- end -}}
+
+          {{- else -}}
+
+            {{/* Fallback: list sibling pages in the docs section */}}
+            {{- with .CurrentSection -}}
+            <small>{{ .Title }}</small>
+            <ul>
+              {{- range .RegularPages -}}
+              <li>
+                <a href="{{ .RelPermalink }}"
+                   {{- if eq .RelPermalink $.RelPermalink }} aria-current="page"{{ end -}}>
+                  {{- .LinkTitle -}}
+                </a>
+              </li>
+              {{- end -}}
+            </ul>
+            {{- end -}}
+
+          {{- end -}}
+
+        </nav>
+      </aside>
+
+
+      {{/* ── Main content ──────────────────────────────────────────────── */}}
+
+      <article>
+
+        {{- if .Description -}}
+        <hgroup>
+          <h1>{{ .Title }}</h1>
+          <p>{{ .Description }}</p>
+        </hgroup>
+        {{- else -}}
+        <h1>{{ .Title }}</h1>
+        {{- end }}
+
+        {{ .Content }}
+
+        {{- if or .PrevInSection .NextInSection -}}
+        <div data-role="prev-next">
+          {{- with .NextInSection -}}
+          <a href="{{ .RelPermalink }}" rel="prev">
+            <small>← Previous</small>
+            <span>{{ .LinkTitle }}</span>
+          </a>
+          {{- end -}}
+          {{- with .PrevInSection -}}
+          <a href="{{ .RelPermalink }}" rel="next">
+            <small>Next →</small>
+            <span>{{ .LinkTitle }}</span>
+          </a>
+          {{- end -}}
+        </div>
+        {{- end -}}
+
+      </article>
+
+
+      {{/* ── Right TOC ────────────────────────────────────────────────── */}}
+
+      {{- with .TableOfContents -}}
+      <aside data-toc>
+        <small>On this page</small>
+        {{ . }}
+      </aside>
+      {{- end -}}
+
+    </div>
+  </main>
+
+  <footer>
+    <small><a href="/">{{ .Site.Title }}</a> · {{ now.Format "2006" }}</small>
+  </footer>
+{{ end }}