fix(awareness): make the 250 km range ring readable on any ground

The Global Context range ring was a Cesium ellipse outline at 1 px in
rgba(98,181,255,0.72). WebGL ignores larger ellipse outline widths, so it
could not be thickened, and it vanished over desert and snow and under the
FLIR, NVG and CRT styles.

Draw it as a polyline instead: a 4 px line with a 2 px Context-blue core
inside a 1 px dark halo on each side (PolylineOutlineMaterialProperty).
The halo gives luminance contrast on light and dark ground, which the
monochrome styles keep. The ring sits at the same 2.5 km height and
follows the subject on the same 250 m threshold. Positions come from a
new awarenessRangeRingPositions helper that stays within 50 m of 250 km.

Style values live in AWARENESS_RANGE_RING_STYLE (awareness/policy.js).
Adds rangeRing.test.mjs (geometry at equator, Sahara, Greenland and the
antimeridian; contrast; entity shape; follow threshold) and a
CURRENT-STATE note.

src/layers/awareness/rendering.js

@@ -10,9 +10,60 @@ import {
   AWARENESS_RADIUS_M,
 } from '../../data/militaryAwarenessEngine.js';
 import { bearingBetweenCoordinates } from '../../cockpitMath.js';
-import { AWARENESS_PAGE_SIZE, CONTEXT_RIM_HEIGHT_M } from './policy.js';
+import {
+  AWARENESS_PAGE_SIZE,
+  AWARENESS_RANGE_RING_STYLE,
+  CONTEXT_RIM_HEIGHT_M,
+} from './policy.js';
 import { formatAwarenessDirection } from './presentation.js';
 
+/**
+ * A closed ring of positions `radiusM` along the surface from `groundCenter`,
+ * raised to `heightM`. Each bearing steps over a sphere with the ellipsoid's
+ * radius of curvature in that direction (Euler's formula), which keeps the
+ * ring within a few metres of the true geodesic radius at 250 km.
+ * @param {Cesium.Cartesian3} groundCenter Ring centre on the ellipsoid.
+ * @param {number} radiusM Surface radius in metres.
+ * @param {number} heightM Height of the ring above the ellipsoid.
+ * @param {number} [segments] Chords around the ring.
+ * @returns {Cesium.Cartesian3[]} `segments + 1` positions; the last repeats the first.
+ */
+export function awarenessRangeRingPositions(
+  groundCenter,
+  radiusM,
+  heightM,
+  segments = AWARENESS_RANGE_RING_STYLE.segments,
+) {
+  const { latitude, longitude } =
+    Cesium.Cartographic.fromCartesian(groundCenter);
+  const { x: a, z: b } = Cesium.Ellipsoid.WGS84.radii;
+  const e2 = 1 - (b * b) / (a * a);
+  const sinLat = Math.sin(latitude);
+  const cosLat = Math.cos(latitude);
+  const w = 1 - e2 * sinLat * sinLat;
+  const meridional = (a * (1 - e2)) / (w * Math.sqrt(w));
+  const primeVertical = a / Math.sqrt(w);
+  const positions = [];
+  for (let i = 0; i <= segments; i++) {
+    const bearing = ((i % segments) / segments) * Cesium.Math.TWO_PI;
+    const cosB = Math.cos(bearing);
+    const sinB = Math.sin(bearing);
+    const angular =
+      radiusM * ((cosB * cosB) / meridional + (sinB * sinB) / primeVertical);
+    const lat = Math.asin(
+      sinLat * Math.cos(angular) + cosLat * Math.sin(angular) * cosB,
+    );
+    const lon =
+      longitude +
+      Math.atan2(
+        sinB * Math.sin(angular) * cosLat,
+        Math.cos(angular) - sinLat * Math.sin(lat),
+      );
+    positions.push(Cesium.Cartesian3.fromRadians(lon, lat, heightM));
+  }
+  return positions;
+}
+
 export function createRendering({
   state: layerState,
   services,
@@ -292,15 +343,19 @@ export function createRendering({
     );
     const key = `${subject.layerId}:${subject.id}`;
     if (layerState.visual?.key === key) {
-      // Large ellipse geometry is more reliable as a ConstantPositionProperty.
-      // Move it only after a meaningful displacement so live tracking does not
-      // churn ten large geometries for sub-pixel motion every 750 ms.
+      // Move the ring only after a meaningful displacement so live tracking
+      // does not rebuild its geometry for sub-pixel motion every 750 ms.
       if (
         Cesium.Cartesian3.distance(layerState.visual.center, groundCenter) >=
         250
       ) {
+        const positions = awarenessRangeRingPositions(
+          groundCenter,
+          AWARENESS_RADIUS_M,
+          CONTEXT_RIM_HEIGHT_M,
+        );
         for (const entity of layerState.visual.entities)
-          entity.position.setValue(groundCenter);
+          entity.polyline.positions.setValue(positions);
         Cesium.Cartesian3.clone(groundCenter, layerState.visual.center);
         // Content actually changed — buy exactly one frame instead of holding.
         governorRequestRender('awareness-visual');
@@ -311,17 +366,26 @@ export function createRendering({
 
     const entities = [];
 
+    // A polyline, not an ellipse outline: WebGL draws ellipse outlines at
+    // 1 px whatever `outlineWidth` says, and the ring needs a halo.
+    const ring = AWARENESS_RANGE_RING_STYLE;
     entities.push(
       layerState.viewer.entities.add({
-        position: groundCenter,
-        ellipse: {
-          semiMajorAxis: AWARENESS_RADIUS_M,
-          semiMinorAxis: AWARENESS_RADIUS_M,
-          fill: false,
-          outline: true,
-          outlineColor:
-            Cesium.Color.fromCssColorString('#62b5ff').withAlpha(0.72),
-          height: CONTEXT_RIM_HEIGHT_M,
+        polyline: {
+          positions: new Cesium.ConstantProperty(
+            awarenessRangeRingPositions(
+              groundCenter,
+              AWARENESS_RADIUS_M,
+              CONTEXT_RIM_HEIGHT_M,
+            ),
+          ),
+          width: ring.width,
+          arcType: Cesium.ArcType.NONE,
+          material: new Cesium.PolylineOutlineMaterialProperty({
+            color: Cesium.Color.fromCssColorString(ring.color),
+            outlineColor: Cesium.Color.fromCssColorString(ring.outlineColor),
+            outlineWidth: ring.outlineWidth,
+          }),
         },
       }),
     );