feat(embedding,notes): implement ADR-0056 Blake3 stable hashing and ADR-0057 priority-aware retention

server/src/embedding.rs

@@ -152,7 +152,6 @@ fn fallback_embed(text: &str) -> Vec<f32> {
         return vec;
     }
 
-    use std::hash::{Hash, Hasher};
     for word in words {
         let clean: String = word
             .chars()
@@ -163,18 +162,17 @@ fn fallback_embed(text: &str) -> Vec<f32> {
             continue;
         }
 
-        let mut hasher = std::collections::hash_map::DefaultHasher::new();
-        clean.hash(&mut hasher);
-        let h = hasher.finish();
+        let hash_bytes = blake3::hash(clean.as_bytes());
+        let h = u64::from_le_bytes(hash_bytes.as_bytes()[0..8].try_into().unwrap());
         let idx = (h as usize) % DIM;
         let sign = if (h >> 32) & 1 == 0 { 1.0f32 } else { -1.0f32 };
         vec[idx] += sign;
 
         let chars: Vec<char> = clean.chars().collect();
         for window in chars.windows(3) {
-            let mut h2 = std::collections::hash_map::DefaultHasher::new();
-            window.hash(&mut h2);
-            let hv = h2.finish();
+            let window_str: String = window.iter().collect();
+            let h2_bytes = blake3::hash(window_str.as_bytes());
+            let hv = u64::from_le_bytes(h2_bytes.as_bytes()[0..8].try_into().unwrap());
             let idx2 = (hv as usize) % DIM;
             let s2 = if (hv >> 32) & 1 == 0 { 0.5f32 } else { -0.5f32 };
             vec[idx2] += s2;
@@ -378,4 +376,14 @@ mod tests {
             assert_eq!(embedding.len(), 384);
         }
     }
+
+    #[test]
+    fn test_fallback_embed_deterministic_stability() {
+        let text = "The quick brown fox jumps over the lazy dog";
+        let emb1 = fallback_embed(text);
+        let emb2 = fallback_embed(text);
+        assert_eq!(emb1.len(), 384);
+        assert_eq!(emb1, emb2);
+        assert!((cosine_similarity(&emb1, &emb2) - 1.0).abs() < 1e-5);
+    }
 }