- Drive the columns collection from the `column` tag, recovering 3 columns
authored under Notes/ that the glob missed
- Tag the 6 columns with subject tags; add a subjects collection and a
/subjects/ listing page (newest-first), linked from the footer
- minmax(0,1fr)+min-width:0 and pre-wrap so a long fenced line no longer blows out the grid
- build-time KaTeX via @vscode/markdown-it-katex; ship katex CSS+fonts and link in layouts
