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_pmtiles_builder.rb

@@ -76,6 +76,27 @@ class PmtilesBuilderTest < Minitest::Test
     SH
     File.chmod(0o755, @osmium_ok)
 
+    # Same shape as osmium-ok, but fails on tags-filter specifically (the
+    # first subcommand HikingTrails.extract runs) while still succeeding
+    # on merge — lets a single-country build (which never calls merge)
+    # exercise "hiking-trails extraction failed" without also breaking
+    # offline.pmtiles' own osmium use in a multi-country test.
+    @osmium_fail_tags_filter = File.join(@bin_dir, "osmium-fail-tags-filter")
+    File.write(@osmium_fail_tags_filter, <<~SH)
+      #!/bin/sh
+      echo "osmium $1" >> "$PMTILES_LOG"
+      if [ "$1" = "tags-filter" ]; then
+        echo "boom" >&2
+        exit 1
+      fi
+      prev=""
+      for arg; do
+        if [ "$prev" = "-o" ]; then echo fake > "$arg"; fi
+        prev="$arg"
+      done
+    SH
+    File.chmod(0o755, @osmium_fail_tags_filter)
+
     # Stands in for `java -jar planetiler.jar --osm-path=... --output=OUT`
     # — PLANETILER_JAR itself is irrelevant here (this fake ignores -jar's
     # argument entirely), only JAVA_BIN needs pointing at it.
@@ -188,13 +209,42 @@ class PmtilesBuilderTest < Minitest::Test
     assert_path_exists File.join(@data_dir, "offline.pmtiles")
     refute_path_exists File.join(@data_dir, "offline.tmp.pmtiles")
     assert_equal "done", Maptool::PmtilesBuilder.offline_status[:state]
-    assert_equal ["planetiler --osm-path"], log_lines
+    assert_equal ["planetiler --osm-path", "osmium tags-filter", "osmium add-locations-to-ways", "osmium cat"], log_lines
     assert_equal(
       { "sources" => [{ "code" => "SK", "slug" => "europe/slovakia", "last_modified" => "Wed, 10 Sep 2026 08:00:01 GMT" }] },
       Maptool::PmtilesBuilder.offline_meta(@data_dir),
     )
   end
 
+  def test_build_offline_also_writes_a_hiking_trails_geojson
+    select_regions("SK")
+    thread = Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download)
+    thread.join
+
+    # The fake osmium binary doesn't produce real OPL output, just
+    # confirms the file lands and is valid (empty) GeoJSON — the real
+    # parsing logic is covered by test_hiking_trails.rb against real OPL
+    # fixtures.
+    path = File.join(@data_dir, "hiking_trails.geojson")
+    assert_path_exists path
+    assert_equal({ "type" => "FeatureCollection", "features" => [] }, JSON.parse(File.read(path)))
+  end
+
+  # The actual reasoning: hiking trails are a bonus overlay bolted onto the
+  # existing offline.pmtiles build, not the core map — an osmium hiccup on
+  # this one step shouldn't take down an otherwise-successful build.
+  def test_build_offline_still_succeeds_when_hiking_trails_extraction_fails
+    select_regions("SK")
+    ENV["OSMIUM_BIN"] = @osmium_fail_tags_filter
+    thread = Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download)
+    thread.join
+
+    assert_path_exists File.join(@data_dir, "offline.pmtiles")
+    assert_equal "done", Maptool::PmtilesBuilder.offline_status[:state]
+    refute_path_exists File.join(@data_dir, "hiking_trails.geojson")
+    refute_path_exists "#{File.join(@data_dir, "hiking_trails.geojson")}.tmp"
+  end
+
   # Two countries selected — osmium merge should run before planetiler,
   # each downloaded source recorded in the sidecar meta.
   def test_build_offline_merges_multiple_selected_countries
@@ -203,7 +253,14 @@ class PmtilesBuilderTest < Minitest::Test
     thread.join
 
     assert_equal "done", Maptool::PmtilesBuilder.offline_status[:state]
-    assert_equal ["osmium merge", "planetiler --osm-path"], log_lines
+    assert_equal(
+      [
+        "osmium merge", "planetiler --osm-path",
+        "osmium tags-filter", "osmium add-locations-to-ways", "osmium cat", # SK's own hiking-trails extraction
+        "osmium tags-filter", "osmium add-locations-to-ways", "osmium cat", # AT's own
+      ],
+      log_lines,
+    )
     codes = Maptool::PmtilesBuilder.offline_meta(@data_dir)["sources"].map { |s| s["code"] }
     assert_equal %w[SK AT], codes
   end