Prototype: extract offline hiking trails from the Geofabrik PBF we already have

No custom Planetiler profile needed after all — Maptool::HikingTrails
runs osmium tags-filter r/route=hiking against the same .osm.pbf
PmtilesBuilder already downloads per country for offline.pmtiles, then
osmium cat -f opl (locations_on_ways=true) to get member-way geometry
back as plain text, no separate node-id lookup table to build.

No general OSM tool stitches a route relation into one line (routes
are a semantic grouping, not a geometry type) — parse_opl does that
walk itself, same as the Overpass-based click handler already does
client-side. Kept pure/testable apart from the two osmium subprocess
calls, mirroring OverpassPois.fetch vs .to_geojson.

rake hiking:trails[pbf,out] to try it by hand against a real cached
extract. Not wired into the actual offline build or rendering yet —
still just a prototype, per TODO.adoc.

test/test_hiking_trails.rb

@@ -0,0 +1,103 @@
+require_relative "test_helper"
+require "maptool/hiking_trails"
+
+# No real osmium calls — extract() is a thin subprocess-plus-parse wrapper
+# (same "not independently tested" reasoning OverpassPois.fetch gets), but
+# parse_opl is pure text in, GeoJSON out, and that's the part actually worth
+# protecting: the OPL escaping rules and the way-id -> relation-member
+# stitching are easy to get subtly wrong and osmium isn't installed in every
+# environment this test suite runs in.
+class HikingTrailsTest < Minitest::Test
+  def test_parse_opl_builds_a_multilinestring_from_member_way_geometry
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18
+      w2 Thighway=path Nn11x20.088y49.18,n12x20.089y49.181
+      r1 Troute=hiking,type=route,name=Rysy Mw1@,w2@
+    OPL
+
+    features = Maptool::HikingTrails.parse_opl(opl)["features"]
+
+    assert_equal 1, features.size
+    feature = features.first
+    assert_equal "MultiLineString", feature["geometry"]["type"]
+    assert_equal [[[20.087, 49.179], [20.088, 49.18]], [[20.088, 49.18], [20.089, 49.181]]], feature["geometry"]["coordinates"]
+    assert_equal "Rysy", feature["properties"]["name"]
+  end
+
+  def test_parse_opl_skips_relations_that_are_not_route_hiking
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18
+      r1 Ttype=multipolygon Mw1@outer
+    OPL
+
+    assert_empty Maptool::HikingTrails.parse_opl(opl)["features"]
+  end
+
+  def test_parse_opl_falls_back_to_description_when_there_is_no_name
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18
+      r1 Troute=hiking,type=route,description=Rázc.%20%pri%20%Žabom%20%potoku%20%-%20%Rysy Mw1@
+    OPL
+
+    feature = Maptool::HikingTrails.parse_opl(opl)["features"].first
+
+    assert_equal "Rázc. pri Žabom potoku - Rysy", feature["properties"]["name"]
+  end
+
+  def test_parse_opl_unescapes_commas_and_equals_in_tag_values
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18
+      r1 Troute=hiking,type=route,name=Salt%2c%%20%Pepper%3d%Co Mw1@
+    OPL
+
+    feature = Maptool::HikingTrails.parse_opl(opl)["features"].first
+
+    assert_equal "Salt, Pepper=Co", feature["properties"]["name"]
+  end
+
+  def test_parse_opl_carries_over_colour_network_operator_ref
+    opl = <<~OPL
+      w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18
+      r1 Troute=hiking,type=route,name=Trail,colour=red,network=rwn,operator=KST,ref=0933 Mw1@
+    OPL
+
+    properties = Maptool::HikingTrails.parse_opl(opl)["features"].first["properties"]
+
+    assert_equal "red", properties["colour"]
+    assert_equal "rwn", properties["network"]
+    assert_equal "KST", properties["operator"]
+    assert_equal "0933", properties["ref"]
+  end
+
+  def test_parse_opl_computes_distance_from_member_way_geometry
+    # Roughly 111.2m per 0.001 degree of latitude at the equator.
+    opl = <<~OPL
+      w1 Thighway=path Nn10x0.0y0.0,n11x0.0y0.001
+      r1 Troute=hiking,type=route,name=Trail Mw1@
+    OPL
+
+    distance = Maptool::HikingTrails.parse_opl(opl)["features"].first["properties"]["distance_meters"]
+
+    assert_in_delta 111.2, distance, 1.0
+  end
+
+  def test_parse_opl_drops_a_relation_whose_member_ways_are_all_missing
+    opl = <<~OPL
+      r1 Troute=hiking,type=route,name=Ghost Mw999@
+    OPL
+
+    assert_empty Maptool::HikingTrails.parse_opl(opl)["features"]
+  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
+      r1 Troute=hiking,type=route,name=Trail Mw1@,w999@
+    OPL
+
+    features = Maptool::HikingTrails.parse_opl(opl)["features"]
+
+    assert_equal 1, features.size
+    assert_equal 1, features.first["geometry"]["coordinates"].size
+  end
+end