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.

docs/CURRENT-STATE.md

@@ -4957,6 +4957,7 @@ silently demoting every later lookup for the session.
 - Enabled Data Layer controls report normalized feed health on the button (`LOADING`, `DEGRADED`, `STALE`, `FALLBACK`, or `UNAVAILABLE`) while the metadata line retains the source and reason. A partial CelesTrak group failure keeps the usable catalog and reports `DEGRADED`; a total outage keeps last-good catalog data visible but reports `UNAVAILABLE`.
 - On activation it focuses the nearest currently observed aircraft across the civilian and military feeds, with military winning an exact distance tie; if none are available, it focuses an observed AIS vessel. The aircraft search is deliberately uncapped and one refresh-tick retry handles initially empty feeds. This is an attention-priority navigation shortcut, not a high-risk, affiliation, or threat classification.
 - A selected aircraft, AIS vessel, or mapped installation gets a 250 km **context window** with nearby cohort counts, nearest examples, source labels, and stale/unavailable reasons. It emits `NEARBY` or `UNKNOWN`; no detection, engagement, affiliation, or sensor-activity conclusion is calculated.
+- The window's edge is drawn on the globe as a 250 km range ring: a 4 px polyline with a 2 px Context-blue core inside a 1 px dark halo on each side, 2.5 km above the ellipsoid (`AWARENESS_RANGE_RING_STYLE` in `src/layers/awareness/policy.js`). The halo keeps it legible over bright desert and snow, dark ocean and forest, and under the FLIR, NVG and CRT styles, where the earlier 1 px ellipse outline disappeared. It is a polyline because WebGL draws ellipse outlines at 1 px whatever width is asked for. The ring follows a moving subject once it has moved 250 m.
 - For a selected live aircraft or vessel, the context window refreshes from the existing tracker/feed position every 750 ms, so nearest distances and cohort counts follow the subject without introducing a duplicate poll loop.
 - Aircraft cohort membership uses every locally loaded, selectable contact inside the 250 km window, including a plane hidden only by horizon culling or because its 3D model owns the visual. Counts and navigation therefore do not change with the current camera angle or billboard/model handoff.
 - Nearby examples in the context panel are focus controls: they use their owning layer's existing selection/tracking path, then frame that contact. Static-installation distances use ellipsoidal surface distance so the count matches the ground-projected context disk.