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.

changelog.d/1433-awareness-marker-kilometre.md

@@ -1,3 +1,6 @@
 - Show a Contacts marker 999.5 m or more away as 1.0 km. The on-globe marker
   chose metres before rounding, so 999.6 m read "1000 m" while the panel read
   "1.0 km" (Aditya, @Rohilalala, #1433).
+- Distance labels share one formatter that takes each unit cut after rounding,
+  so the Contacts panel and markers, the drawn-line measure and the Cockpit
+  show 9.96 km as 10 km rather than 10.0 km (#1433).