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.

docs/development.adoc

@@ -101,6 +101,50 @@ source uses — `public/js/dem.js`'s own point-elevation lookups
 (`shadow.js`'s terrain-shadow overlay) read the actual decoded tile
 size rather than assuming one, so this isn't a problem either way.
 
+== Offline hiking trails
+
+The sidebar's "Hiking trails" overlay works online out of the box
+(waymarkedtrails.org raster tiles, `public/js/hiking_trails.js`) —
+clicking a trail fires a one-off Overpass query for its name/colour/
+network/distance. For it to work offline too, `data/hiking_trails.geojson`
+is generated automatically as the last step of every `offline.pmtiles`
+build (`Maptool::HikingTrails`, `lib/maptool/hiking_trails.rb`), straight
+from the *same* per-country `.osm.pbf` files already downloaded for
+`offline.pmtiles` itself — no separate download, no third-party service:
+
+[source,shell]
+----
+osmium tags-filter country.osm.pbf r/route=hiking -o hiking.osm.pbf
+osmium add-locations-to-ways hiking.osm.pbf -o hiking-located.osm.pbf
+osmium cat hiking-located.osm.pbf -f opl,locations_on_ways=true,add_metadata=false -o hiking.opl
+----
+
+`add-locations-to-ways` matters: a plain Geofabrik PBF's ways only carry
+bare node *references*, not coordinates, so skipping this step silently
+produces ways with empty x/y in the OPL output and every route gets
+dropped for having no usable geometry — not an error, just wrong output
+(verified against a real extract before this was caught). No general OSM
+tool stitches a route relation's member ways into one line for you
+(routes are a semantic grouping, not a geometry type), so
+`HikingTrails.parse_opl` does that walk itself — the same thing
+waymarkedtrails.org and this app's own online Overpass click handler
+already do.
+
+Best-effort: a hiking-trails extraction problem on one country's PBF
+logs a warning and is skipped rather than failing the whole
+`offline.pmtiles` build over what's a bonus overlay, not the core map
+(`PmtilesBuilder.extract_hiking_trails`). Coordinates are rounded to 5
+decimal places (~1m, plenty for a rendered line) to keep the combined
+file smaller — for a many-country selection it can still be a sizeable
+GeoJSON (Slovakia alone: ~3000 routes, ~30MB unrounded), fetched
+client-side only once the overlay is actually toggled on, not on page
+load. `rake hiking:trails[pbf,out]` runs the extraction by hand against
+one `.osm.pbf` for testing.
+
+`DELETE /api/offline-tiles` removes this file along with `offline.pmtiles`
+and the Geofabrik cache — there's no separate delete/rebuild route for it,
+since it's not an independent build.
+
 == In-app regeneration
 
 A logged-in user can also generate both files above from the app