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/marker.js

@@ -1,15 +1,13 @@
 import * as Cesium from 'cesium';
 import { positionMarkerGlyph } from './glyphs.js';
 import { COLORS, POSITION_PICK_ID } from './policy.js';
-import { refineHeights } from './groundCast.js';
 import { createHorizonCull } from './view.js';
 
 const SPRITE_ID = 'street-level:marker';
 
-/** On-globe marker for the open image; in terrain mode it stands on bare earth. */
-export function createMarker({ state, parts }) {
+/** On-globe marker for the open image, clamped to the ground. */
+export function createMarker({ state }) {
   const { render, sprites } = state.services;
-  let last = null;
 
   function requestRender() {
     render?.governorRequestRender?.('street-level-marker');
@@ -31,36 +29,10 @@ export function createMarker({ state, parts }) {
     sprites?.registerSpriteCollection?.(SPRITE_ID, state.marker.collection);
   }
 
-  /**
-   * Terrain-mode height (bare earth refined by mesh samples), or null to
-   * clamp. Requests a missing terrain cell or mesh sample as a side effect.
-   */
-  function castHeight(position) {
-    const ground = parts?.groundCaster;
-    if (state.surface !== 'terrain' || !ground) return null;
-    const { lon, lat } = position;
-    const dem = ground.groundAt(lon, lat);
-    if (dem === null) {
-      ground.prepare([[lon, lat]]).then((ready) => {
-        if (ready && last?.position === position) set(position, last.bearing);
-      });
-      return null;
-    }
-    const mesh = parts.meshSampler?.meshAt(lon, lat);
-    if (mesh === undefined) parts.meshSampler?.request([[lon, lat]]);
-    return refineHeights([{ dem, mesh }])[0];
-  }
-
-  // A new mesh sample may move the marker onto (or off) the road surface.
-  parts?.meshSampler?.onSampled(() => {
-    if (last && state.surface === 'terrain') set(last.position, last.bearing);
-  });
-
   /** Move (or create) the marker; a null position removes it. */
   function set(position, bearing) {
     const collection = state.marker.collection;
     if (!collection) return;
-    last = position ? { position, bearing } : null;
     if (!position) {
       collection.removeAll();
       state.marker.billboard = null;
@@ -68,16 +40,7 @@ export function createMarker({ state, parts }) {
       requestRender();
       return;
     }
-    const height = castHeight(position);
-    const cartesian = Cesium.Cartesian3.fromDegrees(
-      position.lon,
-      position.lat,
-      height ?? 0,
-    );
-    const heightReference =
-      height === null
-        ? Cesium.HeightReference.CLAMP_TO_GROUND
-        : Cesium.HeightReference.NONE;
+    const cartesian = Cesium.Cartesian3.fromDegrees(position.lon, position.lat);
     if (!state.marker.billboard) {
       state.marker.billboard = collection.add({
         id: POSITION_PICK_ID,
@@ -87,12 +50,11 @@ export function createMarker({ state, parts }) {
         width: 40,
         height: 40,
         alignedAxis: Cesium.Cartesian3.UNIT_Z,
-        heightReference,
+        heightReference: Cesium.HeightReference.CLAMP_TO_GROUND,
         disableDepthTestDistance: Number.POSITIVE_INFINITY,
         verticalOrigin: Cesium.VerticalOrigin.CENTER,
       });
     } else {
-      state.marker.billboard.heightReference = heightReference;
       state.marker.billboard.position = cartesian;
     }
     state.marker.billboard.rotation = -Cesium.Math.toRadians(bearing || 0);
@@ -115,17 +77,11 @@ export function createMarker({ state, parts }) {
     state.marker.billboard = null;
   }
 
-  /** Re-place the marker after the surface mode changes. */
-  function setSurface() {
-    if (last) set(last.position, last.bearing);
-  }
-
   return {
     ensure,
     set,
     clear: () => set(null),
     setVisible,
-    setSurface,
     destroy,
   };
 }