Retire the single-font rule

One family with weight and size contrast carrying the hierarchy is a
legitimate type system, and in practice the rule mostly punished
minimal pages: it was the loudest cross-rule noise on the fixture
corpus's should-pass columns. Removed from the registry, both engine
paths, the regex page analyzers, and the devtools category map; the
negative assertions stay as resurrection guards, and the text-content
analyzer index base shifts down one with the removal.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cli/engine/rules/checks.mjs

@@ -3911,12 +3911,6 @@ function checkTypography() {
       if (isBrandFontOnOwnDomain(font)) continue;
       findings.push({ type: 'overused-font', detail: `Primary font: ${font} (${Math.round(share * 100)}% of text)` });
     }
-
-    // Single-font check: only one distinct primary font across all text
-    if (fontUsage.size === 1) {
-      const only = [...fontUsage.keys()][0];
-      findings.push({ type: 'single-font', detail: `only font used is ${only}` });
-    }
   }
 
   const sizes = new Set();
@@ -4178,14 +4172,6 @@ function checkPageTypography(doc, win) {
     findings.push({ id: 'overused-font', snippet: `Primary font: ${font}` });
   }
 
-  // Single font
-  if (fonts.size === 1) {
-    const els = doc.querySelectorAll('*');
-    if (els.length >= 20) {
-      findings.push({ id: 'single-font', snippet: `only font used is ${[...fonts][0]}` });
-    }
-  }
-
   // Flat type hierarchy
   const sizes = new Set();
   const textEls = doc.querySelectorAll('h1, h2, h3, h4, h5, h6, p, span, a, li, td, th, label, button, div');