fix(awareness): keep range pulses out of picking and in the style palette

Review follow-up for the sonar pulses (the non-blocking items).

Unpickable pulses: the three pulse lines were polyline entities, which
Cesium always picks, so near-miss and touch picks around them became
unowned hits. They are now drawn by the pulse disc's shader, which
already drew the gradient and is built with allowPicking false. Each
line keeps the polyline outline material's look: 3 CSS px wide with a
1 px dark outline in the ring's colours, measured from the screen-space
gradient of the disc's radius times czm_pixelRatio, as Cesium's contour
material does. Without derivatives only the gradient band is drawn.

Disc follow: with the lines on the disc, it is rebuilt after the subject
moves 500 m instead of 2 km, so the pulses stay concentric with the
ring. A resting disc is hidden and not rebuilt; it catches up when the
pulses wake.

Camera below the disc: the disc sits 2.5 km above the ellipsoid with
faceForward, so from a lower camera it swept across the sky. It is now
hidden while the camera is below that height.

Echo palette: echo brackets keep the pulse blue on the default
detection style and take the style's line colour under NVG, FLIR and
CRT.

Echo test: the detection renderer's echo maths moves into
awarenessEchoBracket and awarenessEchoColor in src/awarenessEcho.js,
tested directly instead of running a regex slice of detection.js
through vm, and the one-sided spread assert is now two-sided.

The positions reuse parameter of awarenessRangeRingPositions and the
pulse segment count had no callers left and are removed.

src/awarenessEcho.js

@@ -109,3 +109,44 @@ export function awarenessEchoAt(layerId, position, nowMs) {
     )
   );
 }
+
+/**
+ * The echo bracket the detection overlay draws around one contact, or null
+ * when it has no echo. `band` is the alpha band, 1 to `steps`, scaled by the
+ * contact's own keyhole `admissionAlpha`; `spreadPx` is how far the echo
+ * sits outside the contact's bracket, 0 as the front arrives, growing to the
+ * field's `spreadPx` as it fades.
+ * @param {string} layerId Owning layer of the contact.
+ * @param {Cesium.Cartesian3} position Contact's world position.
+ * @param {number} nowMs Monotonic clock in milliseconds.
+ * @param {number} admissionAlpha The contact bracket's own fade, 0 to 1.
+ * @param {number} steps Alpha bands the overlay batches by.
+ * @returns {{band: number, spreadPx: number}|null}
+ */
+export function awarenessEchoBracket(
+  layerId,
+  position,
+  nowMs,
+  admissionAlpha,
+  steps,
+) {
+  const echo = awarenessEchoAt(layerId, position, nowMs);
+  if (!(echo > 0.01)) return null;
+  return {
+    band: Math.max(1, Math.ceil(echo * admissionAlpha * steps)),
+    spreadPx: field.echo.spreadPx * (1 - echo),
+  };
+}
+
+/**
+ * The echo's stroke colour: the field's own colour on the default detection
+ * style, and the style's line colour under the others (NVG, FLIR, CRT), so
+ * an echo never stands out of a monochrome palette.
+ * @param {string} styleLine The active detection style's line colour.
+ * @param {boolean} defaultStyle Whether the default detection style is active.
+ * @returns {string|null} Null when no field is set.
+ */
+export function awarenessEchoColor(styleLine, defaultStyle) {
+  if (!field) return null;
+  return defaultStyle ? field.echo.color : styleLine;
+}