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/distanceFormat.js

@@ -0,0 +1,43 @@
+/**
+ * Kilometre decimals for a distance: one below 10 km, none from there on.
+ * The cut is taken on the tenth that prints, so 9.96 km reads 10 km, not 10.0.
+ * @param {number} meters
+ * @returns {0|1}
+ */
+export function kilometreDigits(meters) {
+  return Math.round(meters / 100) < 100 ? 1 : 0;
+}
+
+/**
+ * The unit and precision a distance prints with: metres below 1 km, then
+ * kilometres with `kilometreDigits`. Metres round to `metreStep` (10 for tens),
+ * except below `exactBelowM`, where they stay whole. Each cut is taken after
+ * rounding, so 999.6 m reads 1.0 km rather than 1000 m.
+ * @param {number} meters
+ * @param {{metreStep?: number, exactBelowM?: number}} [options]
+ * @returns {{value: number, unit: 'm'|'km', digits: 0|1}|null} null for a
+ *   distance that is negative or not finite. The value is already rounded to
+ *   `digits`, so toFixed and Intl render it the same way.
+ */
+export function distanceParts(meters, { metreStep = 1, exactBelowM = 0 } = {}) {
+  if (!Number.isFinite(meters) || meters < 0) return null;
+  const step = meters < exactBelowM ? 1 : metreStep;
+  const shownMeters = Math.round(meters / step) * step;
+  if (shownMeters < 1000) return { value: shownMeters, unit: 'm', digits: 0 };
+  const digits = kilometreDigits(meters);
+  const value =
+    digits === 1 ? Math.round(meters / 100) / 10 : Math.round(meters / 1000);
+  return { value, unit: 'km', digits };
+}
+
+/**
+ * Plain `850 m` / `4.6 km` / `12 km` text for labels that are not localized.
+ * @param {number} meters
+ * @param {{metreStep?: number, exactBelowM?: number}} [options]
+ * @returns {string|null}
+ */
+export function formatDistanceText(meters, options) {
+  const parts = distanceParts(meters, options);
+  if (!parts) return null;
+  return `${parts.value.toFixed(parts.digits)} ${parts.unit}`;
+}