GeoJSON without a CRS: 3857 metres read as degrees

• districts.geojson parses fine with 312 features, but L.geoJSON(data).addTo(map) draws nothing and logs nothing.
• map.fitBounds(layer.getBounds()) returns LatLngBounds(20037508, 20037508).
• QGIS reports an invalid layer extent, and the first coordinate pair is [13522390.4, 3669871.9].

1. Read the first coordinate pair and check magnitude; an absolute value above 180 is not a degree.
2. Look for a top-level crs member, but note that Leaflet 1.x and Mapbox GL ignore it.
3. Compute the bbox: x 7.3e6~1.5e7 and y 2.4e6~6.1e6 match China in EPSG:3857.
4. Trace back to the export SQL and check whether ST_Transform(geom, 4326) was applied.
5. If the export cannot be changed, reproject before constructing L.geoJSON, not inside coordsToLatLng.

Fix it at the source: serialize with ST_AsGeoJSON(ST_Transform(geom, 4326)); on the client, walk the coordinates with proj4('EPSG:3857','EPSG:4326', xy) before calling L.geoJSON.

const c = fc.features[0].geometry.coordinates;
const is4326 = Math.abs(c[0]) <= 180 && Math.abs(c[1]) <= 90;
console.log(is4326 ? '经纬度' : '投影坐标，量级 ' + c[0].toExponential(2));
// 13522390.4 -> "投影坐标，量级 1.35e+7"

-- PostGIS：导出前必须转换
SELECT jsonb_build_object(
  'type', 'FeatureCollection',
  'features', jsonb_agg(ST_AsGeoJSON(t, 15)::jsonb)
)
FROM (
  SELECT id, name, ST_Transform(geom, 4326) AS geom FROM districts
) AS t;

• Adding a crs member naming EPSG:3857 and expecting Leaflet to honour it, which it does not.
• Passing a coordsToLatLng callback that guesses units, which silently mangles valid degree data.