Add osmtogeojson fixture generator

README.md

@@ -25,6 +25,8 @@ uv run ty check
 uv run pytest tests/ --import-mode importlib
 ```
 
+For GeoJSON hardening fixtures, see `docs/geojson-fixtures.md`.
+
 ## Usage
 
 ### `API()` constructor

docs/geojson-fixtures.md

@@ -0,0 +1,24 @@
+# GeoJSON Fixtures (osmtogeojson reference)
+
+We use `osmtogeojson` as a reference implementation to generate GeoJSON fixtures
+for relation-heavy cases (multipolygons, routes, boundaries).
+
+This does **not** add a runtime dependency. It is only for generating fixtures.
+
+## Setup
+
+Install Node and the converter:
+
+```bash
+npm install --global osmtogeojson
+```
+
+## Generate a fixture
+
+Given an Overpass JSON response saved to `tests/fixtures/input.json`:
+
+```bash
+node tools/osmtogeojson_fixture.js tests/fixtures/input.json tests/fixtures/output.geojson
+```
+
+Commit the resulting `output.geojson` and use it as a golden file in tests.