On-the-Fly Centroid Normalization Alignment; Precomputed Reproduction Unchanged

Patch release correcting centroid normalization for the raw-text/on-the-fly embedding path. Post embeddings are now normalized before being averaged into retrieval centroids.

This change does not affect the precomputed-embedding path or the documented reproduction workflow, which continues to use the released precomputed embeddings.

No changes are required for reproducing the reported results using the documented procedure.