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/militaryAwarenessEngine.test.mjs

@@ -29,6 +29,9 @@ test('a contact marker moves a distance that rounds to 1,000 m into kilometres',
   assert.equal(formatAwarenessDistance(999.4), '999 m');
   assert.equal(formatAwarenessDistance(999.6), '1.0 km');
   assert.equal(formatAwarenessDistance(4600), '4.6 km');
+  assert.equal(formatAwarenessDistance(9949), '9.9 km');
+  assert.equal(formatAwarenessDistance(9950), '10 km');
+  assert.equal(formatAwarenessDistance(9960), '10 km');
 });
 
 test('awareness cohorts sort nearest-first and do not turn empty data into out of range', () => {