fix(directions): show a 9.96 km route as 10 km, not 10.0 km

routeDistanceText picked one decimal while the raw kilometres were under
10, then rounded them for display, so 9.95 km or more read "10.0 km".
It now picks the unit from the rounded tenth, as it already does at the
metre/kilometre boundary.

docs/CURRENT-STATE.md

@@ -4172,7 +4172,9 @@ animate). Disable clears all state.
 
 At the metre/kilometre boundary, route distance labels use the rounded
 ten-metre value to select the unit, switching to kilometres at 995 metres;
-a 996-metre leg displays as `1.0 km` rather than `1000 m`.
+a 996-metre leg displays as `1.0 km` rather than `1000 m`. Whole kilometres
+start where the label would round to 10.0 km, so a 9.96 km route displays as
+`10 km` rather than `10.0 km`.
 
 **Keys needed: none.** Directions works on a keyless boot, and the layer is off
 until the operator turns it on.