fix(accel): restore f32::EPSILON near-zero-denom guard in cosine_similarity

The SIMD migration weakened the near-zero-norm guard in all four
clawhdf5-accel cosine_similarity backends (scalar/avx2/avx512/neon)
from `denom < f32::EPSILON` to `denom == 0.0`. Vectors with a tiny
but nonzero norm (denom in (0, 1.19e-7)) fell through to dot/denom
and scored as identical instead of maximally dissimilar, diverging
from the pre-SIMD scalar loop's documented fallback behavior.

Restores the epsilon threshold in all four backends so
`1.0 - cosine_similarity(...)` in hnsw.rs::compute_distance
reproduces the old fallback exactly. Adds regression tests in
clawhdf5-accel and clawhdf5-ann locking in the near-zero-norm case.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

crates/clawhdf5-accel/src/avx2.rs

@@ -111,7 +111,7 @@ pub unsafe fn cosine_similarity(a: &[f32], b: &[f32]) -> f32 {
         }
 
         let denom = (norm_a * norm_b).sqrt();
-        if denom == 0.0 { 0.0 } else { dot / denom }
+        if denom < f32::EPSILON { 0.0 } else { dot / denom }
     }
 }
 

crates/clawhdf5-accel/src/avx512.rs

@@ -89,7 +89,7 @@ pub unsafe fn cosine_similarity(a: &[f32], b: &[f32]) -> f32 {
         }
 
         let denom = (norm_a * norm_b).sqrt();
-        if denom == 0.0 { 0.0 } else { dot / denom }
+        if denom < f32::EPSILON { 0.0 } else { dot / denom }
     }
 }
 

crates/clawhdf5-accel/src/lib.rs

@@ -361,6 +361,18 @@ mod tests {
         assert!(approx_eq(cosine_similarity(&a, &b), 0.0, EPSILON));
     }
 
+    #[test]
+    fn test_cosine_near_zero_norm_clamped() {
+        // denom = 1e-4 * 1e-4 = 1e-8, comfortably below f32::EPSILON
+        // (~1.19e-7) but not exactly 0.0 — must still clamp to 0.0 so
+        // callers computing `1.0 - cosine_similarity(...)` treat these
+        // as maximally dissimilar, matching the pre-SIMD scalar guard.
+        let a = [1e-4f32];
+        let b = [1e-4f32];
+        assert_eq!(cosine_similarity(&a, &b), 0.0);
+        assert_eq!(scalar::cosine_similarity(&a, &b), 0.0);
+    }
+
     #[test]
     fn test_cosine_scalar_vs_dispatch() {
         let a: Vec<f32> = (0..384).map(|i| (i as f32).sin()).collect();

crates/clawhdf5-accel/src/neon.rs

@@ -94,7 +94,7 @@ pub unsafe fn cosine_similarity(a: &[f32], b: &[f32]) -> f32 {
     }
 
     let denom = (norm_a * norm_b).sqrt();
-    if denom == 0.0 { 0.0 } else { dot / denom }
+    if denom < f32::EPSILON { 0.0 } else { dot / denom }
 }
 
 /// NEON L2 distance.

crates/clawhdf5-accel/src/scalar.rs

@@ -21,7 +21,7 @@ pub fn cosine_similarity(a: &[f32], b: &[f32]) -> f32 {
         norm_b += y * y;
     }
     let denom = (norm_a * norm_b).sqrt();
-    if denom == 0.0 { 0.0 } else { dot / denom }
+    if denom < f32::EPSILON { 0.0 } else { dot / denom }
 }
 
 pub fn batch_cosine(query: &[f32], vectors: &[&[f32]], results: &mut [(usize, f32)]) {