fix: keep exact clutter selectors from removing majority-content containers

[mvanhorn]

Summary

Pages whose main content lives inside a container with an ad-like class (like <app-root class="ad">) no longer come back empty. The exact-selector clutter pass now skips removing an ambiguous ad-like element when it holds the large majority of the page's text, while explicit clutter (#comments, footer, dialogs, nav) and unsafe tags (script, style, noscript, iframe) stay removable as before.

Why this matters

The reporter's page in #283 extracts to zero words. The root cause: EXACT_SELECTORS' .ad entry matches the SPA's root element app-root class="ad", which holds essentially all of the page text, and the display:none print copy is removed by hidden-element handling, so nothing survives. Any site whose framework root or wrapper happens to carry an ad-like class hits the same total-loss failure, and there is no retry path that disables exact selectors.

The guard is deliberately narrow: it only protects elements whose class matches ambiguous ad-like tokens, only when they hold >=80% of the main content's text (minimum 200 chars), and never protects unsafe tags or high-confidence clutter selectors, so existing fixtures are unaffected.

Testing

• New fixture + expected output pair issues--283-clutter-selector-removes-content reproduces the reporter's page shape (SPA root with class="ad", hidden print copy, large inline script): extraction goes from 0 words to the full article, and leaked script HTML is asserted absent.
• TZ=UTC npx vitest run: 20 files, 371 tests pass.

Fixes #283