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/registry/antipatterns.mjs

@@ -28,16 +28,6 @@ const ANTIPATTERNS = [
     skillSection: 'Typography',
     skillGuideline: 'overused fonts like Inter',
   },
-  {
-    id: 'single-font',
-    category: 'slop',
-    scopes: ['type'],
-    name: 'Single font without hierarchy',
-    description:
-      'Only one font family is used for the entire page. A single family can work when weight and size contrast carry the hierarchy; otherwise pair a distinctive display font with a refined body font.',
-    skillSection: 'Typography',
-    skillGuideline: 'only one font family for the entire page',
-  },
   {
     id: 'flat-type-hierarchy',
     category: 'slop',