Function cosine_similarity 

pub fn cosine_similarity(a: &[f32], b: &[f32]) -> f32

Compute cosine similarity between two embedding vectors.

Returns a value in [-1.0, 1.0]:

• 1.0 = identical direction
• 0.0 = orthogonal (unrelated)
• -1.0 = opposite direction

Returns 0.0 for empty vectors or vectors of different lengths.