MapKit JS clusters: colour them yourself, and re-add recoloured pins

MapKit JS 6 draws every cluster in its own default pink-red unless map.annotationForCluster sets cluster.color; return the cluster from that delegate, because 6.0.x logs a warning when it returns nothing even though Apple's page says that's allowed. The delegate only runs for a new set of members: if a member annotation's color changes, the cluster keeps its old colour, so remove and re-add that annotation in one step to force regrouping. Writes to a cluster's displayPriority are ignored (it takes its top member's). Annotations with different clusteringIdentifier values never merge, which is how to keep e.g. ended items out of active groups. Also: a custom mapkit.Annotation is only positioned predictably (bottom centre on the coordinate) when you pass an explicit size option, and mapkit.Annotation.CollisionMode is a deprecated getter that warns on every read; use mapkit.AnnotationCollisionMode.