fix(flights): start new aircraft at their delayed display position

The fleet renders behind real time, so a newly seen aircraft is displayed at its fix projected back to that delayed time. Its billboard was created at the raw fix instead, so it appeared ahead of where the next fleet tick put it and then jumped back about one render delay of travel before moving forward.

src/data/flights.test.mjs

@@ -168,6 +168,77 @@ test('nonempty OpenSky payload with zero usable rows cannot prove share target a
   }
 });
 
+test('a newly seen aircraft appears at its delayed position, not ahead of it', async () => {
+  // The fleet renders RENDER_DELAY_SEC behind real time, so a fresh contact's
+  // first displayed position is its fix projected back to that delayed time.
+  // Creating the billboard at the raw fix drew it ahead until the next fleet
+  // tick, which then jumped it back about one delay of travel.
+  const added = [];
+  const billboardCollection = {
+    show: false,
+    add(options) {
+      const billboard = { ...options };
+      added.push(billboard);
+      return billboard;
+    },
+    remove() {},
+  };
+  const viewer = { camera: { positionCartographic: null }, scene: {} };
+  _setTrackedFlightRefreshStateForTest({
+    icao24: 'seed00',
+    entity: null,
+    billboard: { show: false, position: Cesium.Cartesian3.fromDegrees(0, 0, 0) },
+    billboardCollection,
+    viewer,
+    meta: { rawLat: 0, rawLon: 0, onGround: false },
+    tracked: false,
+  });
+  const icao24 = 'fresh1';
+  const fixLon = -97.6;
+  const fixLat = 30.3;
+  const speedMps = 250;
+  const nowSec = Math.floor(Date.now() / 1000);
+  const realFetch = globalThis.fetch;
+  globalThis.fetch = async (url) => {
+    if (!String(url).startsWith('/api/flights')) {
+      return { ok: true, status: 200, json: async () => ({ ac: [] }) };
+    }
+    return {
+      ok: true,
+      status: 200,
+      headers: { get: () => null },
+      json: async () => ({
+        time: nowSec,
+        states: [[
+          icao24, 'NEW123 ', 'United States', nowSec - 2, nowSec - 2,
+          fixLon, fixLat, 10_668, false, speedMps, 90, 0, null, 10_700,
+          null, null, null, 5,
+        ]],
+      }),
+    };
+  };
+  try {
+    await flightsLayer.update(viewer);
+    const billboard = added.find((entry) => entry.id === icao24);
+    assert.ok(billboard, 'the new contact gets a billboard');
+    const shown = Cesium.Cartographic.fromCartesian(billboard.position);
+    const fix = Cesium.Cartesian3.fromDegrees(fixLon, fixLat, 10_700);
+    const behindM = Cesium.Cartesian3.distance(
+      Cesium.Cartesian3.fromRadians(shown.longitude, shown.latitude, 10_700),
+      fix,
+    );
+    // Eastbound, so the delayed position is west of the fix by the travel
+    // between the delayed time and the fix (about 28 s).
+    assert.ok(Cesium.Math.toDegrees(shown.longitude) < fixLon, 'drawn behind the fix');
+    assert.ok(
+      behindM > speedMps * 25 && behindM < speedMps * 31,
+      `expected about ${speedMps * 28} m behind the fix, got ${Math.round(behindM)} m`,
+    );
+  } finally {
+    globalThis.fetch = realFetch;
+  }
+});
+
 test('flights poll refreshes tracked callsign/FL/kts and marks a missed poll STALE', async () => {
   const icao24 = 'a1b2c3';
   const entity = { gevLabelModel: { title: 'OLD', details: [] } };