Switch to 100% server-side map clustering

Removes client-side Leaflet MarkerCluster library in favor of
server-side clustering at all zoom levels:
- Zoom 1-8: Density dots
- Zoom 9-15: Server-generated numbered clusters
- Zoom 16+: Individual property markers

This prevents the visual issue where server-returned clusters were
being re-clustered by the client into a single merged marker.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wp-content/plugins/mls-by-hansonxyz/includes/class-mls-cluster.php

@@ -336,15 +336,12 @@ class MLS_Cluster {
             return $this->get_density_data($where_sql, $values, $zoom, $center_lat, $total, self::DENSITY_DOT_SPACING);
         }
 
-        // Zoom 12-15: Numbered clusters (or individual if low count)
+        // Zoom 9-15: Always use server-side clusters (let server handle grouping)
         if ($zoom <= self::ZOOM_CLUSTER_MAX) {
-            if ($total <= self::MAX_INDIVIDUAL_MARKERS) {
-                return $this->get_individual_markers($where_sql, $values, $total);
-            }
             return $this->get_cluster_data($where_sql, $values, $zoom, $center_lat, $total);
         }
 
-        // Zoom 16+: Individual markers
+        // Zoom 16+: Individual markers (no clustering needed at this zoom)
         return $this->get_individual_markers($where_sql, $values, $total);
     }