fix(rhumb-bearing): return NaN for coincident points

packages/turf-rhumb-bearing/index.ts

@@ -54,6 +54,10 @@ function rhumbBearing(
  * var d = p1.rhumbBearingTo(p2); // 116.7 m
  */
 function calculateRhumbBearing(from: number[], to: number[]) {
+  // Coincident points have no defined bearing (matches Geodesy reference impl)
+  if (from[0] === to[0] && from[1] === to[1]) {
+    return NaN;
+  }
   // φ => phi
   // Δλ => deltaLambda
   // Δψ => deltaPsi

packages/turf-rhumb-bearing/test.ts

@@ -46,6 +46,18 @@ test("bearing", (t) => {
     );
   });
 
+  // Coincident points have no defined bearing — must return NaN, not 0
+  // Regression test for https://github.com/Turfjs/turf/issues/2478
+  const coincident = point([5, 5]);
+  t.ok(
+    isNaN(rhumbBearing(coincident, coincident)),
+    "coincident points return NaN"
+  );
+  t.ok(
+    isNaN(rhumbBearing(coincident, coincident, { final: true })),
+    "coincident points return NaN (final bearing)"
+  );
+
   t.throws(() => {
     rhumbBearing(point([12, -54]), "point");
   }, "invalid point");