refactor(street-level): drop FOLLOW and Google 3D ground casting

FOLLOW is gone: no setFollow, follow state, map-stack availability or
handoff listener. Opening a photo still frames it with one flight through
the application's deferred navigation (begin/reassert), so a slow photo
never overrides newer navigation and tracking is released as before.

Ground casting is gone: groundCast.js, meshSampler.js, the surface mode,
the draped-to-cast swap and remesh. Coverage lines are always draped
GroundPolylinePrimitives (ClassificationType.BOTH); cones and the marker
clamp to the ground and keep the horizon cull. The terrain/meshFloor/ground
services and meshFloor.visibleTilesetLoaded are no longer needed.

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

src/layers/streetLevel/providers/mapillary/index.js

@@ -143,12 +143,6 @@ export function createMapillaryProvider({ source }) {
           parts.sequences.rerender();
         },
 
-        /** Core surface mode changed: redraw lines and cones draped or cast. */
-        setSurface() {
-          parts.coverage.setSurface();
-          parts.sequences.rerender();
-        },
-
         coverageStats() {
           return {
             count: parts.coverage.sequenceCount(),