leaflet_markercluster forces clustering on all Leaflet maps on the same page

Problem/Motivation

The submodule leaflet_markercluster unconditionally overrides Drupal.Leaflet.prototype.add_features in modules/leaflet_markercluster/leaflet_markercluster.drupal.js. The Leaflet library attaches leaflet_markercluster/leaflet-markercluster-drupal per block whenever at least one map on the page has map_settings.leaflet_markercluster.control = true. Because the override lives on the prototype and has no per-instance guard, every Leaflet map on that page is routed through L.MarkerClusterGroup, ignoring each map's own leaflet_markercluster.control value. Result: clustering is either off on all maps or on for all maps — it cannot be configured per block/view/formatter.

Steps to reproduce

1. Create a page (or node) that renders two Leaflet maps/blocks (e.g. two leaflet_map view blocks or two node fields using the Leaflet formatter).
2. In block/formatter A) enable "Marker Cluster" (leaflet_markercluster.control = true) with e.g. disableClusteringAtZoom: 8.
3. In block/formatter B) disable "Marker Cluster" (leaflet_markercluster.control = false).
4. Verify drupalSettings.leaflet contains both maps with the expected control values (true / false).
5. Load the page. Expected: map A clusters, map B renders individual markers. Actual: both maps cluster. If A is false and B is true, both cluster as well. Only when both are false (library not attached) does clustering stay off on both.

Proposed resolution

Keep a reference to the original add_features before overriding, and inside the new implementation delegate back to the original when the current map instance does not have clustering enabled:

(function ($, Drupal) {
  var original_add_features = Drupal.Leaflet.prototype.add_features;

  Drupal.Leaflet.prototype.add_features = function (features, initial) {
    if (!this.map_settings
      || !this.map_settings.leaflet_markercluster
      || !this.map_settings.leaflet_markercluster.control) {
      return original_add_features.call(this, features, initial);
    }
    // existing clustering implementation ...
  };
})(jQuery, Drupal);

This makes clustering opt-in per map instance without changing the public API.

Remaining tasks

* Review and commit the JS fix in leaflet_markercluster.drupal.js.

User interface changes

None.

API changes

None. The signature of Drupal.Leaflet.prototype.add_features(features, initial) is unchanged. Only behaviour changes: when map_settings.leaflet_markercluster.control is falsy for a given instance, the base leaflet implementation is used instead of the clustering one.

Data model changes

None. No schema, config schema, or stored settings are changed. Existing leaflet_markercluster settings (control, options, excluded, include_path) are honoured as documented.