feat(globe): a follow handle names its target

FollowHandle.target() returns the followed point, checked by the contract
on both engines. The contract's model check now waits for ready() before
looking for the model's pixels: the magenta point and sprites could fill
the region before the glTF loaded (qa-globe-contract cesium 73/73,
navara 79/79).

src/globe/cesium/follow.js

@@ -188,6 +188,8 @@ export function createCesiumFollow(viewer, { commanded }) {
     if (!handle) {
       handle = {
         native: entity,
+        target: () =>
+          displayPosition(entity, viewer.clock.currentTime, undefined),
         stop() {
           if (viewer.trackedEntity === entity) viewer.trackedEntity = undefined;
         },
@@ -306,6 +308,13 @@ export function createCesiumFollow(viewer, { commanded }) {
     const state = { entity: null, proxy: null, options, handle: null };
     state.handle = {
       native: options.native,
+      target() {
+        try {
+          return state.options.position();
+        } catch {
+          return undefined;
+        }
+      },
       stop() {
         if (own !== state) return;
         if (viewer.trackedEntity === state.entity)

src/globe/contract.js

@@ -150,6 +150,8 @@
  * @typedef {object} FollowHandle
  * @property {() => void} stop
  * @property {(next: Partial<FollowOptions>) => void} retarget
+ * @property {() => (Cartesian3 | undefined)} target Where the followed
+ *   target is now (what the follow centres).
  * @property {any} [native]
  * @property {object} [label] FollowOptions.label.
  *

src/globe/follow.js

@@ -350,6 +350,13 @@ export function createFollowController({
         label: options.label,
         offset: null,
         previous: null,
+        target() {
+          try {
+            return handle.options.position();
+          } catch {
+            return undefined;
+          }
+        },
         stop: () => end(handle),
         retarget(next) {
           const updated = normalizeFollowOptions({ ...options, ...next });

src/globe/testing/contractSuite.mjs

@@ -1209,8 +1209,10 @@ export function createContractSuite(
                 count++;
             return count;
           };
+          // ready() turns true once the glTF has loaded (the magenta point
+          // and sprites above can already fill the region before then).
+          await until(() => models.ready(), 'the model set ready()');
           await until(() => modelPixels() > 150, 'the model drawn');
-          ensure(models.ready(), 'a drawn model set is not ready()');
         } finally {
           for (const set of sets) set.destroy();
         }
@@ -2572,6 +2574,11 @@ export function createContractSuite(
           );
           await until(() => offCentre() <= 3, 'the follow centring its target');
           near(range(), Math.hypot(1000, 400), 5, 'follow range at viewFrom');
+          // target(): where the followed target is now.
+          ensure(
+            cartesian3.distance(handle.target(), at(step)) < 1,
+            'following().target() is not the target',
+          );
           // 2 s of motion (at least 120 frames), centred within 3 px.
           const began = performance.now();
           const firstStep = step;