Fix detector URL and advisory handling

Recover joined URL arguments without splitting local paths, derive advisory behavior from registry severity across consumers, inspect readable linked CSS in URL scans, and report only the dominant primary font.

AI assistance disclosure: Implemented and verified with Codex under maintainer direction.

cli/engine/registry/antipatterns.mjs

@@ -233,7 +233,7 @@ const ANTIPATTERNS = [
     // rather than a failure. It fires only on the AI saturation pattern, not on
     // ordinary prose. Advisory findings are surfaced separately, never counted
     // as failures, and skipped by the design hook unless a project opts in.
-    advisory: true,
+    severity: 'advisory',
     name: 'Em-dash overuse',
     description:
       'Em-dash saturation in body copy is an AI cadence tell. Advisory only: humans use em-dashes legitimately, so this fires only on saturation — at least 8 em-dashes (— or --) at a density near one per 500 characters of body text — never on a long article that uses a few. Prefer commas, colons, periods, or parentheses.',
@@ -588,9 +588,10 @@ function getAntipattern(id) {
 // Advisory rules are detected and reported, but never treated as failures:
 // the CLI lists them under a separate "Advisory" section, they do not affect
 // exit codes or the failure count, and the design hook skips them by default.
-// The set is derived from the registry so a rule only needs `advisory: true`.
+// `severity` is the canonical registry field. The runtime finding serializer
+// derives its `advisory: true` compatibility/output flag from this set.
 const ADVISORY_RULE_IDS = new Set(
-  ANTIPATTERNS.filter(rule => rule.advisory === true).map(rule => rule.id),
+  ANTIPATTERNS.filter(rule => rule.severity === 'advisory').map(rule => rule.id),
 );
 
 function isAdvisoryRule(id) {