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/data/routeSteps.test.mjs

@@ -236,6 +236,8 @@ test('distance and duration formatting', () => {
   assert.equal(formatRouteDistance(8), '8 m');
   assert.equal(formatRouteDistance(846), '850 m');
   assert.equal(formatRouteDistance(1234), '1.2 km');
+  assert.equal(formatRouteDistance(996), '1.0 km');
+  assert.equal(formatRouteDistance(9960), '10 km');
   assert.equal(formatRouteDistance(21479), '21 km');
   assert.equal(formatRouteDistance(-1), '');
   assert.equal(formatRouteDuration(40), '40 s');