fix: Update calc clusters to happen after Idle event - reduces issue 563

[ronlv10]

Summary

This PR updates the custom-marker-clustering example to delay cluster rendering until the map becomes idle, rather than reacting to every bounds_changed event during zoom.

Motivation

While investigating issue #563, I found that the aggressive re-clustering during zoom gestures may contribute to a rare but severe bug where the zoom level resets unexpectedly (sometimes to zero). This is especially noticeable with a large number of markers. See comment.

By aligning the behavior with how Google’s official [js-markerclusterer](https://github.com/googlemaps/js-markerclusterer)(https://github.com/googlemaps/js-markerclusterer) handles clustering (listening to the idle event), we reduce the likelihood of triggering this issue and improve performance during zoom gestures.

[usefulthink]

That seems reasonable.
Although the clustering (or rather, retrieving the clusters) is pretty fast, it makes total sense to debounce that a bit, and waiting for the idle event is a great way to do that.

[usefulthink]

Let me know if we should merge this. It sounds like you're still investigating other things regarding the now infamous #563 issue.

[ronlv10]

I think you can merge. I don't investigate it more as I proved that it comes from google maps js