Switch to on-the-fly aggregations for large stac-geoparquet files

Above a certain threshold, visualizing individual items isn't really useful:

I've experimented with on-the-fly quadkey aggregation for another project, and think it could be implemented in **stac-map** as an alternative over a certain item count / zoom level. Wouldn't have to be quadkey, could be another tiling/grid/hex scheme.

No implementation file or test is named. Start by locating the stac-map visualization path and its handling of item counts and zoom levels; compare individual-item rendering with the proposed on-the-fly tiling, grid, or hex aggregation, with done defined as thresholded aggregation for large STAC GeoParquet views.

Written by the indexing model from the issue text.