fix: v0.12.2 — ISS-003 route leg lines now follow the curved great-circle path (greatCircleArc, 25 sampled points via sim slerp) instead of a straight two-point chord; e2e asserts >2 coordinates in the routes source

lib/mapData.ts

@@ -1,5 +1,6 @@
 import type { StateSnapshot } from './game/state';
 import { formatFuel } from './format';
+import { positionAt } from './sim/movement';
 
 // Pure GeoJSON builders for the map's dynamic layers (v0.11.0, package model
 // v0.12.0): one Point feature per plane, one LineString per FLYING plane's
@@ -67,9 +68,27 @@ export interface AirportCoord {
   lon: number;
 }
 
+// Segments used to sample a route leg's great-circle arc (v0.12.2): 25 coordinates
+// total — endpoints exact, 23 intermediate points so the drawn line matches the
+// curved path the plane actually flies (same slerp as lib/sim/movement.ts).
+export const ROUTE_ARC_SEGMENTS = 24;
+
+// Great-circle arc between two airports as [lon, lat] coordinates: positionAt at
+// t = i/SEGMENTS for i in 0..SEGMENTS. Pure — endpoints are exactly the airport
+// coords (positionAt returns them verbatim at t=0 and t=1).
+export function greatCircleArc(from: AirportCoord, to: AirportCoord): [number, number][] {
+  const coords: [number, number][] = [];
+  for (let i = 0; i <= ROUTE_ARC_SEGMENTS; i++) {
+    const pos = positionAt(from.lat, from.lon, to.lat, to.lon, i / ROUTE_ARC_SEGMENTS);
+    coords.push([pos.lon, pos.lat]);
+  }
+  return coords;
+}
+
 // One LineString per FLYING plane — its current leg from locationIcao to dest_icao.
 // Grounded planes draw nothing; a flying plane missing either endpoint from the
-// dataset is skipped, never guessed.
+// dataset is skipped, never guessed. The line is sampled along the great circle
+// (greatCircleArc) so it matches the curved flight path instead of a straight chord.
 export function routesGeoJson(
   state: StateSnapshot | null,
   airportsByIcao: ReadonlyMap<string, AirportCoord>,
@@ -85,10 +104,7 @@ export function routesGeoJson(
       type: 'Feature',
       geometry: {
         type: 'LineString',
-        coordinates: [
-          [from.lon, from.lat],
-          [to.lon, to.lat],
-        ],
+        coordinates: greatCircleArc(from, to),
       },
       properties: { planeId: p.id },
     });