Color-code, cluster and add icons to POI waypoints

A park4night/OsmAnd POI export (e.g. a p4n2gpx area export) tags every
waypoint with a category and a per-category color/icon in its <extensions>
block, all of which was previously thrown away — every waypoint rendered as
the same plain red dot, unreadable once a file has a few thousand of them.

Maptool::Gpx now captures category/color/icon (waypoint_style), matched by
namespace URI rather than local element name: a p4n2gpx file carries both
<osmand:icon> and <locus:icon> side by side, and REXML strips prefixes from
Element#name, so both report as just "icon" — matching on that alone was
silently picking the (useless) Locus drawable path instead of the OsmAnd
category key. Regression test added for this.

On the map: waypoints are colored by category (falling back to the plain
track color for anything uncategorized), a small hand-drawn icon set
(public/js/icons.js's POI_ICONS, rasterized to SDF sprites at runtime) draws
a glyph on top for the categories it covers, the points source is clustered
so a dense export collapses into count bubbles instead of a solid blob, and
a legend in the side panel lists whichever categories are currently visible.

Splitting each file's GeoJSON into a lines source and a points source
(rather than one mixed source with a geometry-type filter per layer) was
needed for clustering, since MapLibre's cluster support only understands a
source made entirely of points.

lib/maptool/gpx.rb

@@ -39,6 +39,7 @@ module Maptool
     end
 
     def self.waypoint_feature(wpt)
+      ext = waypoint_style(wpt)
       {
         "type" => "Feature",
         "geometry" => { "type" => "Point", "coordinates" => coordinate(wpt) },
@@ -47,11 +48,44 @@ module Maptool
           "name" => child_text(wpt, "name"),
           "description" => child_text(wpt, "desc"),
           "time" => child_text(wpt, "time"),
+          "category" => child_text(wpt, "type"),
+          "color" => ext[:color],
+          "icon" => ext[:icon],
         },
       }
     end
     private_class_method :waypoint_feature
 
+    OSMAND_NAMESPACE = "https://osmand.net"
+    private_constant :OSMAND_NAMESPACE
+
+    # POI exports (park4night via p4n2gpx, OsmAnd) tag each waypoint with a
+    # category (<type>) and, in a vendor <extensions> block, a per-category
+    # color and an OsmAnd icon key (e.g. "amenity_parking") — used to
+    # color/icon-code the map instead of every waypoint rendering as the same
+    # plain dot. Matched by namespace *URI*, not just local element name
+    # (unlike point_extensions below, which only cares about local names):
+    # a p4n2gpx export carries both an <osmand:icon> (a category key we can
+    # actually render) and a <locus:icon> (a Locus-app-specific drawable
+    # path) side by side — both strip down to the same local name "icon",
+    # so matching on that alone picks whichever happens to come last.
+    def self.waypoint_style(wpt)
+      values = {}
+      extensions = wpt.elements["extensions"]
+      return values unless extensions
+
+      extensions.elements.each do |el|
+        next unless el.namespace == OSMAND_NAMESPACE
+
+        case el.name
+        when "color" then values[:color] = el.text&.strip
+        when "icon" then values[:icon] = el.text&.strip
+        end
+      end
+      values
+    end
+    private_class_method :waypoint_style
+
     def self.track_feature(trk)
       lines = []
       all_points = []