feat: brute-force cosine vector index

tests/vector_tests.rs

@@ -0,0 +1,14 @@
+use alpha_id::vector::VectorIndex;
+
+#[test]
+fn cosine_topk_ranks_nearest_first() {
+    let vi = VectorIndex::from_vectors(vec![
+        vec![1.0, 0.0],   // id 0
+        vec![0.0, 1.0],   // id 1
+        vec![0.9, 0.1],   // id 2
+    ]);
+    let hits = vi.top_k(&[1.0, 0.0], 2);
+    assert_eq!(hits[0].0, 0);
+    assert_eq!(hits[1].0, 2);
+    assert!(hits[0].1 > hits[1].1);
+}