refactor(street-level): trim dead paths, duplicated labels and test-only API

- visibleBbox: no view-rectangle fallback or grid option; its one caller
  always ranges the rays
- tilesForBbox returns the tiles; normalizeBbox takes arrays only
- viewerHost: no try/catch around adapter calls that catch for themselves
- coverage: counts come from the drawn sequences; no sequenceCount export
- one keyStatusLabel for the pill and the layer list
- no constructor source check, double-init guard or selectSequence /
  clearSequence on the layer

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

src/testSupport/streetLevelFakes.mjs

@@ -103,6 +103,5 @@ export function rayCamera({
       const hit = Cesium.IntersectionTests.rayEllipsoid(ray, ellipsoid);
       return hit ? Cesium.Ray.getPoint(ray, hit.start) : undefined;
     },
-    computeViewRectangle: () => undefined,
   };
 }