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/app/layers/streetLevel.js

@@ -1,6 +1,7 @@
 import { createStreetLevelLayer } from '../../layers/streetLevel/index.js';
 import { createMapillaryProvider } from '../../layers/streetLevel/providers/mapillary/index.js';
 import { createGoogleProvider } from '../../layers/streetLevel/providers/google/index.js';
+import { createKartaViewProvider } from '../../layers/streetLevel/providers/kartaview/index.js';
 import * as sprites from '../../data/spriteOrder.js';
 import * as picking from '../../data/pickRegistry.js';
 import * as input from '../../data/inputOwnership.js';
@@ -10,12 +11,16 @@ import * as render from '../../renderGovernor.js';
 /** The browser Maps key the scene was started with (src/app/scene.js). */
 const googleApiKey = () => globalThis.window?.__GOOGLE_MAPS_API_KEY__ || null;
 
-/** Mapillary and Google Street View, once the Mapillary source is supplied; else none. */
+/**
+ * Mapillary, Google Street View and KartaView, once the Mapillary source is
+ * supplied; else none.
+ */
 function defaultProviders(sources) {
   if (sources.mapillary == null) return [];
   return [
     createMapillaryProvider({ source: sources.mapillary }),
     createGoogleProvider({ getApiKey: googleApiKey }),
+    createKartaViewProvider(),
   ];
 }