Cut weak interest matches and add the rating-driven affinity signal (step 2)

An interest matches an article only when it is in the top three by z and
z >= 1.0, so the Matches line, the stored rows and the weights agree. The
new bounded affinity signal blends each matched interest's rating-derived
weight, gated on attributable ratings like feed affinity.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K9PrjtUS16PAQve8D4bHgc

src/curate/rank.rs

@@ -62,6 +62,11 @@ fn calculate_utility_for(
             ("quality", configured.quality, 1.0),
             ("fit", configured.fit, 1.0),
             ("knn", configured.knn, candidate.signals.knn_gate),
+            (
+                "affinity",
+                configured.affinity,
+                candidate.signals.affinity_gate,
+            ),
             ("interest", configured.interest, 1.0),
             ("feed", configured.feed, candidate.signals.feed_gate),
             ("triage", configured.triage, 1.0),
@@ -342,7 +347,7 @@ mod tests {
         }
         assert!(!b.signals.weights.contains_key("interest"));
         assert!(
-            (a.signals.weights["knn"] / a.signals.weights["quality"] - (0.15 * 0.5) / 0.40).abs()
+            (a.signals.weights["knn"] / a.signals.weights["quality"] - (0.10 * 0.5) / 0.40).abs()
                 < 1e-9
         );
     }