fix(annotations): label a 996 m line or route as 1.0 km, not 1000 m

Placed route, arrow and drawn-line labels picked metres before rounding
to ten metres, so anything from 995 m to 1 km read "1000 m", even where
the draw hint for the same line read "1.0 km". The label now rounds
first, then picks the unit, and does the same at the 10 km cut, where
9,960 m read "10.0 km".

src/annotations/annotationEngine.test.mjs

@@ -730,3 +730,18 @@ test('only an explicit navigation request permits resolving distant annotation t
   assert.deepEqual(received.map(options => options.allowDistant), [false, true]);
   engine.destroy();
 });
+
+test('a drawn line that rounds up to the next unit is labelled in it, not as 1000 m or 10.0 km', async (t) => {
+  installAnimationFrameStubs(t);
+  const labels = [];
+  const renderer = { add(anno) { labels.push(anno.label); }, update() {}, remove() {}, sync() {}, destroy() {} };
+  const engine = createAnnotationEngine({ viewer: {}, renderer });
+  const metresPerDegree = (6371.0088 * 1000 * Math.PI) / 180;
+  for (const metres of [994, 996, 9940, 9960]) {
+    await engine.annotate([
+      { type: 'route', manual: true, path: [[0, 0], [0, metres / metresPerDegree]] },
+    ]);
+  }
+  assert.deepEqual(labels, ['990 m', '1.0 km', '9.9 km', '10 km']);
+  engine.destroy();
+});