Starter: pub validate_query and clearer NSW reciprocal prune #57

Two small starter/docs gaps from working through the course. Happy to send a PR.

1. validate_query is not usable from DataFusion

vector-db-starter/core/src/dataset.rs keeps validate_query as pub(crate). match_vector_order in vector-db-starter/datafusion/src/lib.rs therefore cannot call it and has to re-check dimension, finiteness, and cosine norm. Make it pub and reuse it in the matcher.

2. “both endpoints so I5 still holds” is ambiguous

course/src/rust-04-nsw.md (NSW construction / Checkpoint 2) says to remove a rejected edge from both endpoints so I5 still holds. “I5” is not defined there, so it is unclear what invariant must survive.

Rephrase to the actual rule: the graph is reciprocal. If prune drops a -> b, also drop b -> a. Updating only one adjacency list leaves a one-sided edge.

I can implement these if useful.