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.test.mjs

@@ -137,15 +137,3 @@ test('a rejected key gates like a missing one and is named as rejected', () => {
     false,
   );
 });
-
-test('the surface mode defaults to draped and passes through', () => {
-  assert.equal(
-    composeUIState({ ...base, providers: [provider()] }).surface,
-    'draped',
-  );
-  assert.equal(
-    composeUIState({ ...base, providers: [provider()], surface: 'terrain' })
-      .surface,
-    'terrain',
-  );
-});