fix(agent): deterministic hybrid ranking; don't reinforce zero-score filler

test_hebbian_activation_boost failed intermittently. Root causes, all in the
query path:

- normalize_scores mapped a set of identical scores — including the
  single-candidate case — to 0.0, so a lone perfect match contributed nothing
  to the fused score. Identical positive scores now normalise to 1.0 (all
  equally the best match); identical non-positive scores stay 0.0.
- merge_vector_keyword sorted a HashMap's entries by score alone and then
  truncated, so which ties survived varied from run to run; hybrid_search had
  the same problem in its final sort. Both now break ties by index.
- hybrid_search applied the Hebbian boost to every returned record, including
  the zero-score filler that pads the list when fewer than k records match.
  With random tie-breaking a filler record could collect as many boosts as the
  real hit. Only records with a positive fused score are reinforced now.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/clawhdf5-agent/src/search.rs

@@ -113,13 +113,24 @@ impl HDF5Memory {
                 }
             })
             .collect();
+        // Ties broken by index so results (and therefore which records get
+        // boosted) don't depend on HashMap iteration order upstream.
         results.sort_by(|a, b| {
             b.score
                 .partial_cmp(&a.score)
                 .unwrap_or(std::cmp::Ordering::Equal)
+                .then(a.index.cmp(&b.index))
         });
 
-        let hit_indices: Vec<usize> = results.iter().map(|r| r.index).collect();
+        // Only reinforce records that actually matched. When fewer than `k`
+        // records are relevant, the rest of the list is zero-score filler;
+        // boosting it would teach the store that arbitrary records are
+        // important just because they were nearby in iteration order.
+        let hit_indices: Vec<usize> = results
+            .iter()
+            .filter(|r| r.score > 0.0)
+            .map(|r| r.index)
+            .collect();
         self.apply_hebbian_boost(&hit_indices);
         self.flush().ok();