feat(compare): trends explorer with England line; gap-honest axis; series regression guard

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/ComparisonChart.module.css

@@ -65,3 +65,9 @@
         min-width: 0;
     }
 }
+
+.chartNote {
+  font-size: 0.78rem;
+  color: var(--text-muted);
+  margin: 0.5rem 0 0;
+}