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/paper/single.html

@@ -1,40 +0,0 @@
-{{ 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 }}