docs: fix mod--newline typo, restructure Theming, document articles-list / articles-filter / image modifiers

[Arty2]

Why

The README's Theming docs had two problems and three vocabularies of modifier were absent from user-facing docs entirely.

1. Typo. README and config.default.toml both spelled mod-newline with one dash. The actual selector is .mod--newline (two dashes), so the documented form silently no-op'd for anyone who copy-pasted it. All other mod--* modifiers in the codebase use two dashes — mod-newline was the lone exception, and it was wrong.

2. Misleading table. The 4-column "Theming" table (themes / colour schemes / layout mods / font mods) put independent toggles on the same row, implying e.g. that theme--default requires mod--newline + mod--font-serif. Per the directive: don't display as a table unless rows and columns make sense to combine.

3. Undocumented vocabularies. Three groups of modifier existed in code but not in the README:

   • The 8 display classes on {{< articles-list class="…" >}} (preformatted, cluster, no-{taxa,types,tags,time,summary,text,gap}).
   • The {{< articles-filter >}} shortcode itself, its class / tagfilter parameters, the sticky helper, and how it pairs with the articles-filterable activation class.
   • The image URL-fragment syntax (#blend, #screen, #multiply, #shadow) processed by layouts/partials/render-image.html and the markdown image render hook.

What

README.md + config.default.toml: mod-newline → mod--newline.

README.md "Theming" section: the 4-column table is replaced by one bulleted list per category (Themes, Colour scheme, Layout modifiers, Font modifiers). Each list reads top-to-bottom; combinations are obvious without misleading row alignment.

README.md "Article-listing modifiers" (new): table with one row per display class — rows are independent here, so a table is the right shape.

README.md "Article filter" (new): documents {{< articles-filter >}}, the class and tagfilter parameters, the sticky modifier (a generic .sticky helper at assets/styles/screen.css:2160 applied via class="sticky"), the typical pairing with articles-list class="articles-filterable", and the noscript fallback showing per-taxon post counts via data-count.

README.md "Image modifiers" (new): documents #blend / #screen / #multiply / #shadow fragment syntax, where the parsing happens (layouts/partials/render-image.html lines 5–7, 51, 62), and the caveat that {{< include >}} does NOT process fragments — use plain markdown image syntax or {{< figure >}}.

Test plan

• grep -rn "mod-newline" (one-dash) returns zero matches in the repo
• Each documented modifier name resolves to a real selector / parameter / fragment keyword in code:
  • Theming mod--* → assets/styles/screen.css:166, 884, 895, 899, 903, 997
  • articles-list display classes → assets/styles/screen.css:2900–2950
  • articles-filter class + tagfilter → layouts/shortcodes/articles-filter.html:3, 5
  • sticky → assets/styles/screen.css:2160
  • image fragments → assets/styles/screen.css:1324–1342 + layouts/partials/render-image.html
• cd exampleSite && hugo --gc --minify clean
• npx html-validate@^9 … exit 0
• Visual sanity: render the README and confirm the Theming section reads as four flat lists and each new section appears with its own table.

Docs-only — no code changes.

---

Generated by Claude Code