KS78: Recency tie-breaker for score equality (#13)

crates/shrimpk-memory/src/echo.rs

@@ -1617,13 +1617,25 @@ impl EchoEngine {
 
         // 7c6. Score inflation cap (KS69, KS76 Track 3): prevent unbounded boost stacking
         // Raised from 0.35 to 0.50 to give temporal + importance boosts headroom.
+        // NOTE: Step 7c7 (recency epsilon) follows this cap and may exceed it
+        // by up to ~3e-5. This is intentional — the epsilon only breaks ties,
+        // never meaningful score differences.
         for result in &mut results {
             let max_allowed = result.similarity as f64 + 0.50;
             if result.final_score > max_allowed {
                 result.final_score = max_allowed;
             }
         }
 
+        // 7c7. KS78: Recency tie-breaker (#13) — after all boosts and caps, add a
+        // negligible epsilon derived from created_at so newer memories win ties.
+        for result in &mut results {
+            if let Some(entry) = store.get(&result.memory_id) {
+                let recency_epsilon = (entry.created_at.timestamp_micros() as f64) * 1e-18;
+                result.final_score += recency_epsilon;
+            }
+        }
+
         // 7d. Re-sort by final_score (similarity + hebbian boost)
         results.sort_by(|a, b| {
             b.final_score
@@ -3616,9 +3628,12 @@ mod tests {
 
         assert!(results.len() >= 2, "Should have at least 2 results");
 
-        // Find both memories in results
+        // Find both memories in results (the Meta memory also mentions "Google",
+        // so match the Google-only memory by excluding results that mention "Meta")
         let meta_result = results.iter().find(|r| r.content.contains("Meta"));
-        let google_result = results.iter().find(|r| r.content.contains("Google"));
+        let google_result = results
+            .iter()
+            .find(|r| r.content.contains("Google") && !r.content.contains("Meta"));
 
         assert!(meta_result.is_some(), "Meta memory should surface");
         assert!(google_result.is_some(), "Google memory should surface");