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/essay/single.html

@@ -0,0 +1,40 @@
+{{ define "header" }}
+  <header>
+    <h1>{{ .Title }}</h1>
+    {{ with .Description }}<p data-abstract>{{ . }}</p>{{ end }}
+    {{- $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 }} · {{ end -}}
+      {{- with .Params.author }}{{ . }}{{ end -}}
+    </p>
+    {{- end }}
+    {{- with .GetTerms "tags" }}
+    <nav data-role="tag-cloud" aria-label="Tags">
+      {{- range . }}
+      <a href="{{ .Permalink }}" data-tag="{{ .Name }}">{{ .Name }}</a>
+      {{- end }}
+    </nav>
+    {{- end }}
+  </header>
+{{ end }}
+
+{{ define "content" }}
+  <article role="main">
+    {{ .Content }}
+    {{- 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>
+{{ end }}