feat: RRF and cross-segment max-score dedupe

tests/fusion_tests.rs

@@ -0,0 +1,31 @@
+use alpha_id::fusion::{rrf_merge, cross_segment_dedupe};
+use alpha_id::model::Candidate;
+
+fn cand(code: &str, seg: usize, rerank: Option<f32>) -> Candidate {
+    Candidate { icd_code: code.into(), alpha_text: code.into(),
+        segment_idx: seg, lexical: None, semantic: None, rerank }
+}
+
+#[test]
+fn rrf_merges_two_rankings_by_reciprocal_rank() {
+    let lex = vec![0usize, 1, 2];   // entry indices ranked
+    let sem = vec![2usize, 0, 3];
+    let merged = rrf_merge(&lex, &sem, 60);
+    // entry 0 appears rank0 in lex and rank1 in sem -> highest
+    assert_eq!(merged[0], 0);
+    assert!(merged.contains(&3));
+}
+
+#[test]
+fn cross_segment_keeps_max_score_and_all_sources() {
+    let cands = vec![
+        cand("E11.9", 0, Some(0.4)),
+        cand("E11.9", 2, Some(0.9)),
+        cand("I10.90", 1, Some(0.5)),
+    ];
+    let mut merged = cross_segment_dedupe(cands);
+    merged.sort_by(|a, b| b.1.partial_cmp(&a.1).unwrap());
+    assert_eq!(merged[0].0.icd_code, "E11.9");
+    assert_eq!(merged[0].1, 0.9);                       // max
+    assert_eq!(merged[0].0.source_segments, vec![0, 2]); // all sources
+}