feat: add GeoJSON layers with below anchors and style slots

[André L. (WipeAir)]

Adds GeoJSON as a layer source: <Fill data> and <Line data> project GeoJSON into the same paint pipeline as tile features, and a <GeoJson> composite draws fill, outline and a pin per Point feature. Any layer can take below="<kind>" to paint just beneath the first layer of that canonical kind, and Neutral, Light and Dark gain belowRoads and belowBuildings slots, so custom geometry such as a city centre can sit under the street network. fit="data" fits the viewport to markers plus GeoJSON bounds. Rings are rewound so holes render under the default fill rule, and malformed features are skipped with a GEOJSON_INVALID warning. Includes docs in docs/geojson.md and a new city-centre golden; all existing goldens are unchanged.