Round every heading readout through one shared helper

The transit, ALPR, vessel, CCTV and flight/military context fixes each
wrapped 359.5° to north with an inline Math.round(x) % 360. They now call
roundHeadingDegrees in src/data/headingDegrees.js, which also wraps negative
and multi-turn bearings. normalizeHeading moves there with it, and
cockpitMath re-exports it so existing imports keep working. It now adds a
turn only to a negative remainder, so a bearing just under half a degree
keeps its precision.

The awareness, Local ADS-B, cockpit wind, cockpit heading and Street Level
readouts use the same helper. The Cockpit's TRACK ACQUIRED signal rounded
without wrapping and could read COURSE 360°; it now uses
formatHeadingReadout, which shows --- for a track that is not finite.

changelog.d/1454-voice-context-heading-north.md

@@ -1,2 +1,5 @@
 - Describe a selected flight or military contact on 359.5° or more to the
   voice assistant as heading 0°, not 360° (Aditya, @Rohilalala, #1454).
+- Heading and bearing readouts share one rounding helper, so the Cockpit's
+  TRACK ACQUIRED course no longer reads 360°, and a negative bearing wraps
+  instead of printing a minus sign (#1454).