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/policy.js

@@ -8,6 +8,7 @@ export const POSITION_PICK_ID = 'sl:pos';
 export const PROVIDER_COLORS = Object.freeze({
   mapillary: '#05cb63',
   google: '#4285f4',
+  kartaview: '#635bff',
 });
 
 /** Colours every provider shares: the selection highlight and the marker. */