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/uiState.js

@@ -49,7 +49,6 @@ export function composeUIState({
   providers,
   street,
   sequence,
-  surface = 'draped',
 }) {
   const active = providers.filter((p) => p.on);
   const { keyRequired, keyRejected, ...coverage } =
@@ -71,6 +70,5 @@ export function composeUIState({
     legend,
     sequence: { ...sequence },
     street: { ...street },
-    surface,
   };
 }