refactor: rename content types to semantic taxonomy

- vault → notes (PKM-exported content)
- posts → articles (short-form, no TOC)
- papers → essays (long-form, with TOC)
- type: post → type: article (posts are just short articles)
- layouts/paper → layouts/essay
- 08a-paper.css → 08a-essay.css
- CSS: fix redundant li resets, remove role="main" from article,
  replace <small> prev/next labels, add console layout
- Update hugo.toml menus, internal URLs, front matter throughout
- Add docs/context.md, docs/css-refactor-plan.md

layouts/notes/single.html

@@ -0,0 +1,95 @@
+{{ define "header" }}
+  <header>
+    {{- if .Description -}}
+    <hgroup>
+      <h1>{{ .Title }}</h1>
+      <p>{{ .Description }}</p>
+    </hgroup>
+    {{- else -}}
+    <h1>{{ .Title }}</h1>
+    {{- end }}
+  </header>
+{{ end }}
+
+{{ define "content" }}
+  <section data-layout="docs">
+
+    <nav aria-label="Notes" data-nav="sidebar">
+      {{- $menuName := .Site.Params.notes_menu | default "notes" -}}
+      {{- $menu := index .Site.Menus $menuName -}}
+      {{- if $menu -}}
+        {{- range $menu -}}
+          {{- if .HasChildren -}}
+          <h3>{{ .Name }}</h3>
+          <ul>
+            {{- range .Children -}}
+            <li>
+              <a href="{{ .URL }}"
+                {{- if $.IsMenuCurrent $menuName . }} aria-current="page"{{ end -}}>
+                {{- .Name -}}
+              </a>
+            </li>
+            {{- end -}}
+          </ul>
+          {{- else -}}
+          <ul>
+            <li>
+              <a href="{{ .URL }}"
+                {{- if $.IsMenuCurrent $menuName . }} aria-current="page"{{ end -}}>
+                {{- .Name -}}
+              </a>
+            </li>
+          </ul>
+          {{- end -}}
+        {{- end -}}
+      {{- else -}}
+        {{- with .CurrentSection -}}
+        <h3>{{ .Title }}</h3>
+        <ul>
+          {{- range .RegularPages -}}
+          <li>
+            <a href="{{ .RelPermalink }}"
+               {{- if eq .RelPermalink $.RelPermalink }} aria-current="page"{{ end -}}>
+              {{- .LinkTitle -}}
+            </a>
+          </li>
+          {{- end -}}
+        </ul>
+        {{- end -}}
+      {{- end -}}
+    </nav>
+
+    <article>
+      {{ .Content }}
+
+      {{- $prevURL := index .Params "prev-url" -}}
+      {{- $prevTitle := index .Params "prev-title" -}}
+      {{- $nextURL := index .Params "next-url" -}}
+      {{- $nextTitle := index .Params "next-title" -}}
+      {{- if or $prevURL $nextURL -}}
+      <footer data-role="prev-next">
+        {{- if $prevURL -}}
+        <a href="{{ $prevURL }}" rel="prev">
+          <span aria-hidden="true">←</span> Previous
+          <span>{{ $prevTitle }}</span>
+        </a>
+        {{- end -}}
+        {{- if and $nextURL $nextTitle -}}
+        <a href="{{ $nextURL }}" rel="next">
+          Next <span aria-hidden="true">→</span>
+          <span>{{ $nextTitle }}</span>
+        </a>
+        {{- end -}}
+      </footer>
+      {{- end -}}
+    </article>
+
+    {{- with .TableOfContents -}}
+    <aside data-toc>
+      <h3>On this page</h3>
+      {{ . }}
+    </aside>
+    {{- end -}}
+
+  </section>
+{{ end }}