Validate every component is_finite() (naming the offending index) and
reject zero-norm vectors before normalizing, via the existing
Err(String) channel — callers already map it through EmbedCallError
{retryable:false} into OmniError::manifest_internal, so no new error
plumbing. The red tests turn green; docs state the finite/non-zero
contract in embeddings.md.
Closes iss-embedding-nan-validation.