st_closest_point(collection, point) — returns the closest vertex from a multi-vertex geometry to a target point. Mirrors ash_geo / PostGIS ST_ClosestPoint. v1 supports:

• st_closest_point(%Geo.MultiPoint{}, %Geo.Point{}) — exact (the collection IS a set of points; the closest "point" is genuinely one of them).
• st_closest_point(%Geo.LineString{}, %Geo.Point{}) — the true closest point on the nearest segment (#279), which may be an interior point of an edge, not just a vertex.

In-memory only — no Cypher pushdown in this slice. Returns nil if the collection is empty or either argument is nil. Always returns the result as a %Geo.Point{srid: 4326} regardless of the collection's internal representation.