fix(reader): short pages kept their whole sidebar

[fiend37]

The bug

trimLeadingNav locates the body by scanning for the first line with >120 characters of visible prose. Pages that have no such line — disambiguation stubs, link lists, error pages — hit firstProse <= 0 and return untouched, so the reader renders the site's entire sidebar as the document.

https://en.wikipedia.org/wiki/Moyu opens with 62 lines of Wikipedia menu (Main page / Contents / Current events / Tools / Appearance…) before reaching **Moyu** may refer to:. The other bail-out — the half-the-document safety valve added in a8994f1 — leaks the same way on a nav-heavy site whose first paragraph sits below the midpoint.

The fix

Both bail-outs now fall back to trimNavRun, which needs no paragraph to anchor on. It walks the run of menu items that opens the page and stops at the first line that could be content:

• The run crosses the filler a menu is padded with — short bare labels between groups (Tools, Appearance), a collapsible group's toggle (- [x] Tools), and logo links carrying no text of their own ([![Wikipedia](…)](…), [](https://www.bbc.com/)).
• It may cross one heading, but only if the menu actually resumes afterwards. Wikis drop the article's own H1 in the middle of the sidebar; a 404 page's ## Error 404 - Oops, the page you're looking for is no longer here is the body, and is kept.
• If the page is nothing but menu, or the run is shorter than a menu, nothing is trimmed.

MediaWiki's Appearance panel ships two sentences long enough to survive the run, so they join the boilerplate list beside the other MediaWiki chrome. Its bare labels (Text, Width, Color) are deliberately left out — those are ordinary section headings on plenty of real articles.

Verification

Ran before/after over 11 pages scraped through r.jina.ai:

Page	Result
en.wikipedia.org/wiki/Moyu	66 lines of menu dropped; opens on From Wikipedia, the free encyclopedia
bbc.com/news/articles/… (404)	menu dropped; opens on the Error 404 heading
BBC News home, Guardian home, Hacker News, MDN, GitHub repo, Naver article, ko.wikipedia article, en.wikipedia Konjac + Mercury	byte-identical

scripts/check-reader.mjs pins both directions — the sidebar must go, a link-dense homepage must keep every headline, a 404 keeps its heading, an all-links page is left alone. Offline, no dependencies, no network; it fails 2/4 against the code this PR replaces, and is wired into CI after tsc.

pnpm lint, pnpm exec tsc -b and pnpm build all pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WmQb9ehzkzYaVtDbMTpHAv