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/ui/cockpitPresentation.js

@@ -5,6 +5,7 @@ import {
   normalizeHeading,
   roundHeadingDegrees,
 } from '../data/headingDegrees.js';
+import { kilometreDigits } from '../data/distanceFormat.js';
 
 export const COCKPIT_HEADING_SLEW_DPS = 28;
 
@@ -120,7 +121,7 @@ export function formatCockpitCoordinate(value, positive, negative) {
  * (`4,2 КМ`): one decimal under 10 km, whole kilometres beyond.
  */
 export function formatCockpitDistance(distanceM) {
-  const digits = distanceM < 10000 ? 1 : 0;
+  const digits = kilometreDigits(distanceM);
   const distance = formatQuantity(distanceM / 1000, 'km', {
     minimumFractionDigits: digits,
     maximumFractionDigits: digits,

src/ui/cockpitPresentation.test.mjs

@@ -1,6 +1,9 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
-import { formatCockpitWindDirection } from './cockpitPresentation.js';
+import {
+  formatCockpitDistance,
+  formatCockpitWindDirection,
+} from './cockpitPresentation.js';
 
 test('wind just west of north reads 0°, not 360°', () => {
   assert.equal(formatCockpitWindDirection(90), 'E · 90°');
@@ -8,3 +11,8 @@ test('wind just west of north reads 0°, not 360°', () => {
   assert.equal(formatCockpitWindDirection(359.4), 'N · 359°');
   assert.equal(formatCockpitWindDirection(359.6), 'N · 0°');
 });
+
+test('a cockpit distance that rounds to 10.0 km drops the decimal', () => {
+  assert.match(formatCockpitDistance(9949), /^9\.9\s?KM$/);
+  assert.match(formatCockpitDistance(9960), /^10\s?KM$/);
+});