fix(flights): keep ground-snap sampling after the clock steps back (#1399)

Recover aircraft ground-height sampling after backward wall-clock changes while preserving sampling budgets.

Contribution by @quanticsoul4772.

src/services/groundSnap.test.mjs

@@ -0,0 +1,62 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import * as Cesium from 'cesium';
+import { createGroundSnap } from './groundSnap.js';
+
+const groundFloor = {
+  cachedMeshFloor: () => null,
+  meshFloorPreferred: () => false,
+  reportValidatedMeshFloorCell: () => {},
+};
+
+/** A viewer whose ground is a loaded 3D tileset, counting height samples. */
+function makeViewer() {
+  const calls = { samples: 0 };
+  const tileset = { show: true, tilesLoaded: true };
+  return {
+    calls,
+    scene: {
+      primitives: { length: 1, get: () => tileset },
+      sampleHeight() {
+        calls.samples += 1;
+        return 1650;
+      },
+      globe: { getHeight: () => 1600 },
+    },
+  };
+}
+
+const atDenverAirport = Cesium.Cartesian3.fromDegrees(-104.6737, 39.8561, 0);
+
+test('a clock that steps back opens a new sample window instead of denying samples until it catches up', (t) => {
+  t.mock.timers.enable({ apis: ['Date'], now: 1_000_000 });
+  const viewer = makeViewer();
+  const snap = createGroundSnap({ groundFloor });
+  const icaos = ['a00001', 'a00002', 'a00003', 'a00004', 'a00005', 'a00006'];
+  for (const icao of icaos) snap.heightFor(viewer, icao, atDenverAirport);
+  assert.equal(viewer.calls.samples, 4, 'four samples per 250 ms window');
+  t.mock.timers.setTime(1_000_000 - 60_000);
+  for (const icao of icaos) snap.heightFor(viewer, icao, atDenverAirport);
+  assert.equal(
+    viewer.calls.samples,
+    6,
+    'the two denied aircraft sample in the new window',
+  );
+});
+
+test('a retry backoff stamped before the clock stepped back does not wait for it to catch up', (t) => {
+  t.mock.timers.enable({ apis: ['Date'], now: 10_000_000 });
+  const viewer = makeViewer();
+  const tileset = viewer.scene.primitives.get(0);
+  const snap = createGroundSnap({ groundFloor });
+  // Tiles still streaming: a miss, backed off for 2 s.
+  tileset.tilesLoaded = false;
+  assert.equal(snap.heightFor(viewer, 'a00001', atDenverAirport), null);
+  tileset.tilesLoaded = true;
+  assert.equal(snap.heightFor(viewer, 'a00001', atDenverAirport), null);
+  assert.equal(viewer.calls.samples, 0, 'the backoff holds');
+  // The clock steps back an hour: the stamp is an hour off, past any backoff.
+  t.mock.timers.setTime(10_000_000 - 3_600_000);
+  assert.equal(snap.heightFor(viewer, 'a00001', atDenverAirport), 1650);
+  assert.equal(viewer.calls.samples, 1);
+});