Wire offline hiking trails into the real build, API, and renderer

data/hiking_trails.geojson now generates automatically as the last
step of every offline.pmtiles build (PmtilesBuilder.extract_hiking_trails,
best-effort — a failure here doesn't fail the whole build), merged
across all selected countries with cross-border dedup by OSM relation
id (HikingTrails.extract_merged). Served at GET /tiles/hiking_trails.geojson
and GET /api/hiking-trails, fetched client-side only once the overlay
is actually toggled on.

hiking_trails.js now prefers this local file whenever it exists,
regardless of which base map (online/offline) is active — same
"prefer local when available" story as HillshadeControl already uses
for terrain. Offline click hit-tests the real vector layer directly
(queryRenderedFeatures) instead of querying Overpass — instant, no
network. Coordinates are rounded to 5dp to keep the combined file
smaller; colour tag values are normalized against a known-safe set
before ever reaching a style paint property.

Verified extract_merged (new: combines multiple countries' extracts,
dedupes overlapping relations) against two real cached country PBFs
on brick directly, not just fixtures.

test/test_hiking_trails.rb

@@ -89,6 +89,43 @@ class HikingTrailsTest < Minitest::Test
     assert_empty Maptool::HikingTrails.parse_opl(opl)["features"]
   end
 
+  def test_parse_opl_carries_the_osm_relation_id_for_cross_country_dedup
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18
+      r1246205 Troute=hiking,type=route,name=Trail Mw1@
+    OPL
+
+    feature = Maptool::HikingTrails.parse_opl(opl)["features"].first
+
+    assert_equal 1246205, feature["properties"]["id"]
+  end
+
+  def test_parse_opl_rounds_coordinates_to_5_decimal_places
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.0876543210y49.1791234567,n11x20.0880000001y49.1800000009
+      r1 Troute=hiking,type=route,name=Trail Mw1@
+    OPL
+
+    coords = Maptool::HikingTrails.parse_opl(opl)["features"].first["geometry"]["coordinates"]
+
+    assert_equal [[[20.08765, 49.17912], [20.088, 49.18]]], coords
+  end
+
+  def test_extract_merged_deduplicates_by_relation_id_across_files
+    feature = ->(id) {
+      { "type" => "Feature", "properties" => { "id" => id, "name" => "Trail #{id}" }, "geometry" => { "type" => "MultiLineString", "coordinates" => [] } }
+    }
+    # Relation 1 shows up in both "country" files (a cross-border trail);
+    # relation 2 only in the second.
+    per_file = { "at.osm.pbf" => [feature.call(1)], "cz.osm.pbf" => [feature.call(1), feature.call(2)] }
+    extract = ->(pbf_path) { { "type" => "FeatureCollection", "features" => per_file.fetch(pbf_path) } }
+
+    merged = Maptool::HikingTrails.extract_merged(%w[at.osm.pbf cz.osm.pbf], extract: extract)
+    ids = merged["features"].map { |f| f["properties"]["id"] }.sort
+
+    assert_equal [1, 2], ids
+  end
+
   def test_parse_opl_ignores_a_dangling_member_reference_but_keeps_the_rest
     opl = <<~OPL
       w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18