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.

test/test_gpx.rb

@@ -41,6 +41,39 @@ class GpxTest < Minitest::Test
     assert_equal [17.2, 48.1], [profile[0]["lon"], profile[0]["lat"]]
   end
 
+  def test_waypoint_captures_category_and_color_from_poi_extensions
+    geojson = Maptool::Gpx.to_geojson(fixture_path("sample_poi.gpx"))
+    free_spot = geojson["features"].find { |f| f["properties"]["name"] == "Free spot" }
+
+    assert_equal "Free motorhome area", free_spot["properties"]["category"]
+    assert_equal "#2E7D32", free_spot["properties"]["color"]
+    assert_equal "tourism_camp_site", free_spot["properties"]["icon"]
+  end
+
+  # Regression test for a real bug: a p4n2gpx export's <extensions> carries
+  # both an <osmand:icon> (a category key this app can render) and a
+  # <locus:icon> (a Locus-app drawable path) side by side. REXML strips
+  # namespace prefixes from Element#name, so both elements report the same
+  # local name "icon" — matching on that alone picked whichever happened to
+  # come last in the file (locus, since it followed osmand), leaving every
+  # waypoint's icon set to a meaningless drawable path that never matched
+  # any category. Must match by namespace URI instead.
+  def test_waypoint_icon_ignores_non_osmand_extensions_with_the_same_local_name
+    geojson = Maptool::Gpx.to_geojson(fixture_path("sample_poi.gpx"))
+    export = geojson["features"].find { |f| f["properties"]["name"] == "p4n2gpx export" }
+
+    assert_equal "amenity_parking", export["properties"]["icon"]
+  end
+
+  def test_waypoint_without_category_or_color_leaves_them_nil
+    geojson = Maptool::Gpx.to_geojson(fixture_path("sample_poi.gpx"))
+    plain = geojson["features"].find { |f| f["properties"]["name"] == "Plain waypoint" }
+
+    assert_nil plain["properties"]["category"]
+    assert_nil plain["properties"]["color"]
+    assert_nil plain["properties"]["icon"]
+  end
+
   def test_kinds_detects_routes
     assert_equal ["route"], Maptool::Gpx.kinds(fixture_path("sample_route.gpx"))
   end