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/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) => {