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/docs/single.html

@@ -58,20 +58,20 @@
       {{- end -}}
     </nav>
 
-    <article role="main">
+    <article>
       {{ .Content }}
 
       {{- if or .PrevInSection .NextInSection -}}
       <footer data-role="prev-next">
         {{- with .NextInSection -}}
         <a href="{{ .RelPermalink }}" rel="prev">
-          <small>← Previous</small>
+          <span aria-hidden="true">←</span> Previous
           <span>{{ .LinkTitle }}</span>
         </a>
         {{- end -}}
         {{- with .PrevInSection -}}
         <a href="{{ .RelPermalink }}" rel="next">
-          <small>Next →</small>
+          Next <span aria-hidden="true">→</span>
           <span>{{ .LinkTitle }}</span>
         </a>
         {{- end -}}