feat(street-level): KartaView as a third, keyless imagery provider

Coverage lines from KartaView's z13-14 JSON tiles (OSM ways with photos,
chained end to start), the photo nearest a clicked line, cones along its
sequence, and a viewer for flat photos and 360° panoramas (a small WebGL
equirectangular renderer, no new dependency) with ‹ › along the sequence.

Mapillary's camera-driven line coverage and sequence cones move to the
provider-neutral core (lineCoverage.js, sequenceCones.js) with the tile
fetch, zoom ladder, colours and pick prefixes injected; Mapillary keeps
thin wrappers and its tests pass unchanged.

KartaView quirks handled: api.kartaview.org (the documented
api.openstreetcam.org serves empty tiles), photo URLs that name storage1
for photos stored elsewhere, and a photo API that answers 429 to more than
about three requests at once (two in flight, retried with backoff).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/streetLevel/selection.js

@@ -28,7 +28,24 @@ export function createSelection({ state, parts }) {
     }
     const entry = state.providers.get(route.providerId);
     if (!entry?.on) return;
-    route.instance.handlePick(route.id);
+    route.instance.handlePick(route.id, {
+      groundPoint: () => groundPointAt(viewer, click.position),
+    });
+  }
+
+  /** Lon/lat of the ground under a screen point, or null (sky, no picker). */
+  function groundPointAt(viewer, screenPosition) {
+    const position = state.services.scenePick?.pickGroundPosition?.(
+      viewer,
+      screenPosition,
+    );
+    const carto = position && Cesium.Cartographic.fromCartesian(position);
+    return carto
+      ? {
+          lon: Cesium.Math.toDegrees(carto.longitude),
+          lat: Cesium.Math.toDegrees(carto.latitude),
+        }
+      : null;
   }
 
   /**