Format every metre/kilometre distance through one shared helper

Directions, the Contacts marker and panel, placed annotation labels, the
drawn-line measure, the Cockpit and formatRouteDistance each chose the unit
and precision on their own, and most chose before rounding: #1450, #1433 and
formatDistanceText or kilometreDigits from src/data/distanceFormat.js, which
cut after rounding. Each keeps its own metre step and rendering.

That also fixes the 10 km cut the Contacts marker and panel, the drawn-line
measure and the Cockpit still had (9,960 m read 10.0 km). The helper returns
kilometres already rounded, so the plain marker and the localized panel no
longer disagree on half-tenths such as 1,450 m.

A negative annotation distance now leaves the label without a distance, and
a drawn line with a coordinate that is not a number shows no measure, rather
than printing 0 m or NaN km. The HUD altitude tag and the spoken view height
keep their own unit choice.

src/annotations/annotationEngine.js

@@ -12,6 +12,7 @@ import {
   sampleGroundHeight,
 } from './annotationResolver.js';
 import { ringCentroid } from './drawMode.js';
+import { formatDistanceText } from '../data/distanceFormat.js';
 
 // Dev convenience: expose the app's Cesium instance for console/preview probing
 // (single shared module instance — avoids dual-Cesium state bugs when testing).
@@ -1572,12 +1573,7 @@ function greatCircleM(a, b) {
 }
 
 function formatDistance(m) {
-  if (!Number.isFinite(m)) return null;
-  // Round before each unit cut: 996 m was printed as "1000 m" and
-  // 9,960 m as "10.0 km".
-  const tens = Math.round(m / 10) * 10;
-  if (tens < 1000) return `${tens} m`;
-  return `${(m / 1000).toFixed(Math.round(m / 100) >= 100 ? 0 : 1)} km`;
+  return formatDistanceText(m, { metreStep: 10 });
 }
 
 function composeRouteLabel(baseLabel, distM, durS, mode, fallback) {

src/annotations/drawMode.js

@@ -21,6 +21,7 @@ import { t, tn } from '../i18n/index.js';
  */
 
 import { greatCircleMeters } from '../geoDistance.js';
+import { formatDistanceText } from '../data/distanceFormat.js';
 
 export const DRAW_SHAPES = Object.freeze(['area', 'line', 'pin']);
 export const MIN_VERTICES = Object.freeze({ area: 3, line: 2, pin: 1 });
@@ -207,11 +208,7 @@ export function ringCentroid(vertices) {
 export function formatMeasure(session) {
   if (!session) return '';
   if (session.shape === 'line') {
-    const m = pathLengthM(session.vertices);
-    // 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`;
+    return formatDistanceText(pathLengthM(session.vertices)) ?? '';
   }
   if (session.shape === 'area') {
     const m2 = ringAreaM2(session.vertices);

src/annotations/drawMode.test.mjs

@@ -346,6 +346,14 @@ test('a line that rounds up to a kilometre reads 1.0 km, not 1000 m', () => {
   assert.match(drawHint(line), /^1\.0 km · /);
 });
 
+test('a line that rounds to 10.0 km reads in whole kilometres', () => {
+  const metresPerDegree = (6371.0088 * 1000 * Math.PI) / 180;
+  const line = createDrawSession('line');
+  addVertex(line, { lon: 0, lat: 0 });
+  addVertex(line, { lon: 0, lat: 9960 / metresPerDegree });
+  assert.equal(formatMeasure(line), '10 km');
+});
+
 test('an area that rounds up to the next unit reads in that unit', () => {
   // A square on the equator whose shoelace area is about `m2`.
   const square = (m2) => {