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.

src/data/headingDegrees.js

@@ -0,0 +1,19 @@
+/** Normalize a heading into the [0, 360) range; 0 when it is not finite. */
+export function normalizeHeading(value) {
+  if (!Number.isFinite(value)) return 0;
+  // Only a negative remainder takes the extra turn, so a positive bearing
+  // keeps its precision (0.49999999999999 must not become 360.5).
+  const remainder = value % 360;
+  const wrapped = remainder < 0 ? remainder + 360 : remainder;
+  return wrapped === 360 ? 0 : wrapped;
+}
+
+/**
+ * Whole degrees for display, 0–359. 359.5° and above rounds to north (0),
+ * and negative or multi-turn values wrap. Null when the value is not finite,
+ * so each caller keeps its own placeholder.
+ */
+export function roundHeadingDegrees(value) {
+  if (!Number.isFinite(value)) return null;
+  return Math.round(normalizeHeading(value)) % 360;
+}