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/cameraVerbs.js

@@ -29,6 +29,7 @@ import {
   holdContinuousRender,
   releaseContinuousRender,
 } from './renderGovernor.js';
+import { heightsFor } from './queries/heights.js';
 
 let _isCockpitActive = null;
 /** °/s by speed word — orbit; pan uses fractions of view height/s. */
@@ -559,25 +560,18 @@ function acquireCorridorFloor(state, takeWhole) {
 export function probeMeshFloorM(scene, cells) {
   const requested = Array.isArray(cells) ? cells.length : 0;
   const empty = { heightM: Number.NaN, sampled: 0, requested };
-  if (!scene || typeof scene.sampleHeight !== 'function' || !requested)
-    return empty;
+  if (!scene || !requested) return empty;
+  const heights = heightsFor(scene);
   let heightM = Number.NaN;
   let sampled = 0;
   for (const cell of cells) {
-    try {
-      const carto = Cesium.Cartographic.fromDegrees(
-        cell.lon,
-        cell.lat,
-        0,
-        _probeCarto,
-      );
-      const height = scene.sampleHeight(carto);
-      if (!Number.isFinite(height)) continue;
-      sampled += 1;
-      heightM = Number.isFinite(heightM) ? Math.max(heightM, height) : height;
-    } catch {
-      /* tiles not ready for this cell */
-    }
+    // A miss is a cell whose tiles are not ready.
+    const height = heights.now(
+      Cesium.Cartographic.fromDegrees(cell.lon, cell.lat, 0, _probeCarto),
+    );
+    if (height === null) continue;
+    sampled += 1;
+    heightM = Number.isFinite(heightM) ? Math.max(heightM, height) : height;
   }
   return { heightM, sampled, requested };
 }