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/policy.js

@@ -26,12 +26,3 @@ export const FILTER_DEFAULT = Object.freeze({ pano: 'all', sinceDays: 0 });
 
 /** Nearest-image search radius in metres (Mapillary's graph API caps it at 50 m). */
 export const NEAREST_RADIUS_M = 50;
-
-/**
- * The only map stack camera follow is offered on: elsewhere a camera at eye
- * height looks at a smeared texture.
- */
-export const FOLLOW_MAP_STACK_ID = 'photoreal';
-
-/** Eye height above the sampled ground when the globe camera follows the viewer. */
-export const FOLLOW_EYE_HEIGHT_M = 2.4;