Added a tundra fringe and ragged polar ice edges to the map generator

tests/heightmap_test.js

@@ -51,6 +51,15 @@ export class HeightmapTest extends TestCase {
     this.assertEqual(field.at(point.x, point.y), 0, "points over the sea stay at zero");
   }
 
+  test_tundra_stays_flat_despite_its_rocky_texture() {
+    const field = new HeightField(fixture(() => "Tundra"));
+    for (let x = -2; x <= 2; x++) {
+      for (let y = -2; y <= 2; y++) {
+        this.assertEqual(field.tileHeight(x, y), 0, "tundra never rises");
+      }
+    }
+  }
+
   test_the_open_sea_stays_flat_beside_a_high_coast() {
     // The distance blend would pull a coastal sea point up towards the mountain
     // next to it; the sea mask pins it to exactly zero instead.