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/fixtures/sample_poi.gpx

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<gpx version="1.1" creator="test">
+  <wpt lat="48.1" lon="17.2">
+    <name>Free spot</name>
+    <type>Free motorhome area</type>
+    <extensions>
+      <osmand:color xmlns:osmand="https://osmand.net">#2E7D32</osmand:color>
+      <osmand:icon xmlns:osmand="https://osmand.net">tourism_camp_site</osmand:icon>
+    </extensions>
+  </wpt>
+  <wpt lat="48.101" lon="17.201">
+    <name>Plain waypoint</name>
+  </wpt>
+  <wpt lat="48.102" lon="17.202">
+    <name>p4n2gpx export</name>
+    <type>Daily parking only</type>
+    <extensions>
+      <osmand:color xmlns:osmand="https://osmand.net">#546E7A</osmand:color>
+      <osmand:icon xmlns:osmand="https://osmand.net">amenity_parking</osmand:icon>
+      <locus:icon xmlns:locus="http://www.locusmap.eu">res/drawable-hdpi/ic_parking_small.png</locus:icon>
+    </extensions>
+  </wpt>
+</gpx>