fix: deterministic fusion tie-order + seeded random eval sampling

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/fusion_tests.rs

@@ -29,3 +29,42 @@ fn cross_segment_keeps_max_score_and_all_sources() {
     assert_eq!(merged[0].1, 0.9);                       // max
     assert_eq!(merged[0].0.source_segments, vec![0, 2]); // all sources
 }
+
+#[test]
+fn fusion_is_deterministic_across_runs() {
+    // Build inputs with deliberate score ties so HashMap iteration order would
+    // matter without the deterministic tiebreaker.
+    // rrf_merge: entries 5 and 10 appear at symmetric positions in both lists,
+    // so their RRF scores are identical.  The tiebreaker must put 5 before 10.
+    let lex = vec![5usize, 10, 20];
+    let sem = vec![10usize, 5, 30];
+    let first = rrf_merge(&lex, &sem, 60);
+    for _ in 0..4 {
+        let run = rrf_merge(&lex, &sem, 60);
+        assert_eq!(first, run, "rrf_merge output changed between runs");
+    }
+    // Verify the tiebreaker: 5 and 10 have equal scores (symmetric positions);
+    // ascending id means 5 should come before 10.
+    let pos5 = first.iter().position(|&x| x == 5).expect("5 missing");
+    let pos10 = first.iter().position(|&x| x == 10).expect("10 missing");
+    assert!(pos5 < pos10, "tiebreaker should put smaller id first: got pos5={pos5} pos10={pos10}");
+
+    // cross_segment_dedupe: two codes with equal score — lexicographic tiebreaker.
+    let cands = vec![
+        cand("Z99.89", 0, Some(0.7)),
+        cand("A00.0",  1, Some(0.7)),
+        cand("M50.00", 2, Some(0.5)),
+    ];
+    let first_dedup = cross_segment_dedupe(cands.clone());
+    for _ in 0..4 {
+        let run = cross_segment_dedupe(cands.clone());
+        let ids: Vec<&str> = run.iter().map(|(d, _)| d.icd_code.as_str()).collect();
+        let first_ids: Vec<&str> = first_dedup.iter().map(|(d, _)| d.icd_code.as_str()).collect();
+        assert_eq!(first_ids, ids, "cross_segment_dedupe output changed between runs");
+    }
+    // Verify tiebreaker: A00.0 < Z99.89 lexicographically, so A00.0 first among ties.
+    assert_eq!(first_dedup[0].0.icd_code, "A00.0",
+        "tiebreaker should put A00.0 before Z99.89 (both score 0.7)");
+    assert_eq!(first_dedup[1].0.icd_code, "Z99.89");
+    assert_eq!(first_dedup[2].0.icd_code, "M50.00");
+}