fix(flights): bound route labels to the scheduled leg

src/data/routePlausible.js

@@ -33,6 +33,18 @@ export function crossTrackKm(lat, lon, lat1, lon1, lat2, lon2) {
   return Math.asin(Math.sin(d13) * Math.sin(b13 - b12)) * EARTH_RADIUS_KM;
 }
 
+/** Signed distance along the great circle from the origin toward the destination. */
+function alongTrackKm(position, origin, destination) {
+  const distance = greatCircleKm(origin, position) / EARTH_RADIUS_KM;
+  const direction =
+    bearingRad(origin.lat, origin.lon, position.lat, position.lon) -
+    bearingRad(origin.lat, origin.lon, destination.lat, destination.lon);
+  return (
+    Math.atan2(Math.sin(distance) * Math.cos(direction), Math.cos(distance)) *
+    EARTH_RADIUS_KM
+  );
+}
+
 const NEAR_ENDPOINT_KM = 130;
 const CROSS_TRACK_KM = 200;
 const LOW_ALT_M = 3700; // ~12 000 ft
@@ -66,6 +78,18 @@ export function routePlausible({
   const near = (point) => greatCircleKm(position, point) < NEAR_ENDPOINT_KM;
   let geomOk = (haveO && near(origin)) || (haveD && near(destination));
   if (!geomOk && haveO && haveD) {
+    const routeKm = greatCircleKm(origin, destination);
+    const alongKm = alongTrackKm(position, origin, destination);
+    // atan2 returns the principal arc in [-πR, πR]. Near an antipodal
+    // destination, its allowed forward extension can cross that branch cut.
+    const segmentAlongKm =
+      alongKm < -CROSS_TRACK_KM
+        ? alongKm + 2 * Math.PI * EARTH_RADIUS_KM
+        : alongKm;
+    // Coincident or antipodal endpoints do not define one minor arc. Keep the
+    // existing loose check when the segment cannot be judged reliably.
+    const segmentKnown =
+      routeKm > 0.001 && routeKm < Math.PI * EARTH_RADIUS_KM - 0.001;
     geomOk =
       Math.abs(
         crossTrackKm(
@@ -76,7 +100,10 @@ export function routePlausible({
           destination.lat,
           destination.lon,
         ),
-      ) < CROSS_TRACK_KM;
+      ) < CROSS_TRACK_KM &&
+      (!segmentKnown ||
+        (segmentAlongKm >= -CROSS_TRACK_KM &&
+          segmentAlongKm <= routeKm + CROSS_TRACK_KM));
   } else if (!geomOk) {
     geomOk = true; // only one endpoint known and not near — cannot judge, allow
   }