fix(vessels): show a 359.7° course as 0°, not 360°

Vessel cards and the AIS HUD rounded the direction without wrapping, so
a vessel reporting no heading and a course over ground of 359.5° or more
read "360°" on its cards and "HDG: 360DEG" in the HUD. The rounded
direction now wraps to 0°.

docs/CURRENT-STATE.md

@@ -503,7 +503,9 @@ those reports arrive as 102.3 knots and 360 degrees. Both are stored as unknown,
 and vessel cards, the HUD and analyst queries show a missing measurement rather
 than a reading. Heading keeps its existing separate check for its own sentinel.
 Genuine readings are unchanged, including a stopped vessel's zero and the
-highest speed and course the fields can encode.
+highest speed and course the fields can encode. Vessel cards and the HUD wrap
+the direction after rounding to whole degrees, so a course of 359.5° or more
+reads `0°` (`0DEG` in the HUD) rather than `360°`.
 
 For an already tracked vessel, AISStream static messages update name, type,
 destination and IMO without changing its position, motion, fix timestamps,

src/data/aisLiveVessels.test.mjs

@@ -1322,6 +1322,27 @@ test('buildVesselCard: heading falls back to course; missing parts are omitted',
   assert.deepEqual(bare.details, []);
 });
 
+test('vessel cards read a course that rounds to north as 0°, not 360°', () => {
+  const card = (course) => buildVesselCard(makeRecord({ heading: null, course })).details[0];
+  assert.equal(card(359.4), 'CONTAINER SHIP · 14.5KT · 359°');
+  assert.equal(card(359.7), 'CONTAINER SHIP · 14.5KT · 0°');
+  assert.equal(
+    buildSelectedVesselCard(makeRecord({ heading: null, course: 359.7 })).details[0],
+    'CONTAINER SHIP · 14.5KT · 0°',
+  );
+});
+
+test('the vessel HUD reads a course that rounds to north as HDG 0DEG', () => {
+  const harness = installWireHarness(undefined, { selectedRecord: null });
+  try {
+    Object.assign(harness.record, { heading: null, course: 359.7 });
+    assert.equal(aisLiveVesselsLayer.selectById(harness.record.mmsi), true);
+    assert.match(harness.hud.textContent, /HDG: 0DEG/);
+  } finally {
+    harness.cleanup();
+  }
+});
+
 test('buildVesselCard: unnamed vessels title as MMSI; long names truncate', () => {
   const unnamed = buildVesselCard(makeRecord({ name: 'VESSEL' }));
   assert.equal(unnamed.title, 'MMSI 353136000');

src/layers/vessels/cards.js

@@ -60,7 +60,10 @@ export function createCards({
     if (record.speed !== null && record.speed !== undefined)
       parts.push(formatSpeed(record.speed));
     const direction = record.heading ?? record.course;
-    if (Number.isFinite(direction)) parts.push(`${Math.round(direction)}°`);
+    // A course of 359.5° and above rounds to north; modulo keeps it from
+    // printing 360°. The selected card and the HUD wrap the same way.
+    if (Number.isFinite(direction))
+      parts.push(`${Math.round(direction) % 360}°`);
     return {
       id: vesselOverlayEntryId(record),
       actionable: Boolean(record?.mmsi),
@@ -91,7 +94,7 @@ export function createCards({
       [
         vesselTypeShort(record) || t('VESSEL'),
         formatSpeed(record.speed),
-        Number.isFinite(direction) ? `${Math.round(direction)}°` : '--°',
+        Number.isFinite(direction) ? `${Math.round(direction) % 360}°` : '--°',
       ].join(' · '),
     ];
     const destination = String(record.destination || '').trim();
@@ -181,7 +184,7 @@ export function createCards({
 
   function formatHeading(heading) {
     return t('{heading}DEG', {
-      heading: Number.isFinite(heading) ? Math.round(heading) : '--',
+      heading: Number.isFinite(heading) ? Math.round(heading) % 360 : '--',
     });
   }