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>

extension/devtools/panel.js

@@ -228,7 +228,6 @@ const FIX_SKILLS = {
   'side-tab':                'distill, polish',
   'border-accent-on-rounded':'distill, polish',
   'overused-font':           'typeset',
-  'single-font':             'typeset',
   'flat-type-hierarchy':     'typeset',
   'gradient-text':           'typeset, distill',
   'ai-color-palette':        'colorize, distill',