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-ann/src/hnsw.rs

@@ -1300,6 +1300,18 @@ mod tests {
         assert!((d - 1.0).abs() < 1e-6); // zero vector -> distance 1
     }
 
+    #[test]
+    fn cosine_near_zero_vector() {
+        // Tiny-but-nonzero, identical-direction vectors: denom is well
+        // below f32::EPSILON but not exactly 0.0. Must still be treated
+        // as a degenerate/unreliable direction (distance 1, "maximally
+        // dissimilar"), not as an exact match (distance 0).
+        let a = vec![1e-4, 1e-4];
+        let b = vec![1e-4, 1e-4];
+        let d = compute_distance(&a, &b, DistanceMetric::Cosine);
+        assert!((d - 1.0).abs() < 1e-6);
+    }
+
     #[test]
     fn insert_into_empty_index() {
         let mut index = HnswIndex::new(4, 16, DistanceMetric::L2);