Temporarily block vector UPDATE for DiskANN and IVF indexes

vec0Update_UpdateVectorColumn writes to flat chunk blobs but does not
update DiskANN graph or IVF index structures, silently corrupting KNN
results. Now returns a clear error for these index types. Rescore
UPDATE is unaffected — it already has a full implementation that
updates both quantized chunks and float vectors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>