fix(draw): show a 999.6 m line as 1.0 km, not 1000 m

formatMeasure checked the raw length against 1000 but printed it rounded, so a drawn line of 999.5 m or more read "1000 m" in the hint. Compare the rounded metres so it moves to kilometres.

changelog.d/1388-draw-line-kilometre.md

@@ -0,0 +1,3 @@
+- Show a drawn line of 999.5 m or more as 1.0 km. The draw hint kept metres
+  below 1000, then rounded 999.6 m up to "1000 m"
+  (Rudy Celekli, @rudycelekli, #1388).

src/annotations/drawMode.js

@@ -208,9 +208,10 @@ export function formatMeasure(session) {
   if (!session) return '';
   if (session.shape === 'line') {
     const m = pathLengthM(session.vertices);
-    return m >= 1000
-      ? `${(m / 1000).toFixed(m >= 10000 ? 0 : 1)} km`
-      : `${Math.round(m)} m`;
+    // Round before the kilometre cut. 999.6 m was printed as "1000 m".
+    const whole = Math.round(m);
+    if (whole < 1000) return `${whole} m`;
+    return `${(m / 1000).toFixed(m >= 10000 ? 0 : 1)} km`;
   }
   if (session.shape === 'area') {
     const m2 = ringAreaM2(session.vertices);

src/annotations/drawMode.test.mjs

@@ -330,3 +330,18 @@ test('longitude unwrapping is continuous and re-wraps to one canonical range', (
   assert.equal(wrapLongitude(180), -180);
   assert.equal(unwrapLongitudes([]).length, 0);
 });
+
+test('a line that rounds up to a kilometre reads 1.0 km, not 1000 m', () => {
+  const metresPerDegree = (6371.0088 * 1000 * Math.PI) / 180;
+  const lat = 999.6 / metresPerDegree;
+  const line = createDrawSession('line');
+  addVertex(line, { lon: 0, lat: 0 });
+  addVertex(line, { lon: 0, lat });
+  const metres = pathLengthM(line.vertices);
+  assert.ok(
+    metres >= 999.5 && metres < 1000,
+    `length should round to 1000 m, got ${metres}`,
+  );
+  assert.equal(formatMeasure(line), '1.0 km');
+  assert.match(drawHint(line), /^1\.0 km · /);
+});