fix(street-level): camera height over Google 3D, cast only when it helps, gate token

From the code review of #768:

- view.js measured the camera's height above ground with globe.getHeight,
  but Google 3D hides the globe, so coverage picked its zoom from the
  ellipsoidal height. Over a high city (Denver, ~1,600 m) a camera 500 m up
  read 2,100 m: coverage stayed at z12/z13, whose tiles are too big to cast,
  and lines stayed draped on the roofs. With the globe hidden, the bare-earth
  height from the ground caster now answers; coverage re-picks its zoom when
  the surface mode changes. Verified in Denver: terrain mode, z14, cast lines.
  The core's surface check uses the same helper (and no longer subtracts the
  2 m lift from the camera height).
- castTile rebuilt a tile even when no line could be cast (terrain proxy
  down, tile too big); it now leaves such tiles as they are.
- The feature gate read Street Level's share token as a literal `0`; it now
  takes it from LAYER_STATE_REGISTRY, since a rebase can reassign it.

New view.test.mjs covers the shown-globe, hidden-globe and no-sample cases.

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

src/layers/streetLevel/index.js

@@ -125,22 +125,23 @@ export function createStreetLevelLayer({
 
   /**
    * Camera height above the bare earth under it, for the surface mode. The
-   * ground comes from the caster's coarse grid; a cold cell is fetched once
-   * and the mode checked again when it lands.
+   * ground comes from the caster's coarse grid; a cold cell near enough to
+   * matter is fetched once and the mode checked again when it lands.
    */
   function cameraHeightForSurface() {
     const carto = state.viewer?.camera?.positionCartographic;
     if (!carto) return null;
-    const fallback = cameraHeightAboveGround(state.viewer);
-    if (!(fallback < SURFACE_WARM_BELOW_M)) return fallback;
+    const caster = parts.groundCaster;
+    const height = cameraHeightAboveGround(state.viewer, {
+      groundAt: caster.groundAt,
+    });
     const lon = (carto.longitude * 180) / Math.PI;
     const lat = (carto.latitude * 180) / Math.PI;
-    const ground = parts.groundCaster.heightAt(lon, lat);
-    if (ground !== null) return carto.height - ground;
-    parts.groundCaster.prepare([[lon, lat]]).then((ready) => {
-      if (ready) scheduleSurfaceSync();
-    });
-    return fallback;
+    if (caster.groundAt(lon, lat) === null && height < SURFACE_WARM_BELOW_M)
+      caster.prepare([[lon, lat]]).then((ready) => {
+        if (ready) scheduleSurfaceSync();
+      });
+    return height;
   }
 
   /**