Ask for heights through one service per scene

About 20 sites called scene.sampleHeight, globe.getHeight,
scene.clampToHeight or the MostDetailed variants directly, each with its own
idea of a miss (undefined, NaN, a throw). They now ask heightsFor(scene) in
src/queries/heights.js:

- now(point, { against, exclude }) answers from what is loaded, or null;
  against is 'surface' (what is drawn) or 'terrain' (loaded terrain only).
- clampNow(position) moves a position onto the drawn surface, or null.
- batch() and clampBatch() wait for the most detailed tiles: null per
  missed point, a rejection when the whole query fails.
- supported() reports { surface, clamp } without allocating.

Cesium still answers now() synchronously, so nothing changes on screen and
nothing is slower: in the live app 1,000 calls cost the same through the
service as direct (terrain 0.5-0.7 ms, surface 2-6 ms a call, run-to-run
noise larger than the difference). Every site already treated a miss as
"hold the last value or fall back"; a renderer that can only answer from a
cache or asynchronously can now plug in at one place.

- A Bhote Koshi evidence point whose terrain sample misses stays at its
  fallback elevation instead of reading the miss as sea level.
- The mesh floor sampler skips a scene without depth-texture support
  instead of spending its budget on probes that throw.
- The world annotation probe's trace reads hit/missed.
- A source scan forbids direct height calls outside the service.

The service is exported as ./queries/heights and documented in
docs/APPLICATION.md.

src/data/geoFileImport.js

@@ -3,6 +3,7 @@ import * as Cesium from 'cesium';
 import { createImportImageBudget } from './geoImportImages.js';
 import { createImportedPins } from './geoImportPins.js';
 import { msg } from '../i18n/marker.js';
+import { heightsFor } from '../queries/heights.js';
 
 /** Largest file accepted, in bytes. */
 const MAX_IMPORT_BYTES = 50 * 1024 * 1024;
@@ -1272,10 +1273,11 @@ async function settleGroundPins(dataSource, viewer, signal) {
     // a buried starting point can make the downward mesh ray miss the surface.
     const ellipsoid =
       viewer.terrainProvider instanceof Cesium.EllipsoidTerrainProvider;
+    const heights = viewer.scene ? heightsFor(viewer.scene) : null;
     for (let i = 0; i < positions.length; i++) {
       const sampled = ellipsoid
         ? 0
-        : viewer.scene?.globe?.getHeight?.(positions[i]);
+        : heights?.now(positions[i], { against: 'terrain' });
       positions[i].height = mesh
         ? (batch[i].groundHeight ??
           Math.max(0, Number.isFinite(sampled) ? sampled : 0))
@@ -1295,7 +1297,7 @@ async function settleGroundPins(dataSource, viewer, signal) {
           indices.push(i);
       if (indices.length) {
         const resolved = await boundedImport(
-          viewer.scene.clampToHeightMostDetailed(
+          heights.clampBatch(
             indices.map((i) => {
               const c = positions[i];
               return Cesium.Cartesian3.fromRadians(