Allow point lookup on vector index

Currently, vector indexes are only uses for nearest neighbor searches (`ORDER BY VEC_DISTANCE(...)`). But it should also be possible to use them for point lookups. If we have an expression like the following:

`SELECT * from t where embedding = "[1.0, 2.0, 3.0]";`

then we should be able to use a vector index to find exact matches.

The issue names no files, tests, or entry points. Start by tracing vector-index handling for ORDER BY VEC_DISTANCE(...) and compare it with equality predicates such as embedding = "[1.0, 2.0, 3.0]"; done means an exact-match query can use the vector index and return matching rows.

Written by the indexing model from the issue text.