perf(awareness): skip echo work for the whole frame while pulses rest

The detection overlay asked the echo field for every visible flight,
military aircraft and vessel on every frame while a subject was
selected, projecting each to the ground before learning the pulses were
resting. It now checks awarenessEchoActive once per frame and builds no
echo paths at rest, and awarenessEchoAt checks the level before the
projection.

A detectionHost test with six echo-eligible flights checks that a
resting frame asks for the level fewer times than there are contacts
and paints no echo; it fails on the per-contact path.

The pulse and echo policy doc comments are attached to their constants
again, and one long comment line is re-wrapped.

src/awarenessEcho.js

@@ -45,6 +45,17 @@ export function getAwarenessEchoField() {
   return field;
 }
 
+/**
+ * Whether any echo can light at `nowMs`: a field is set and its pulses are
+ * running. The overlay checks this once per frame, so resting pulses cost no
+ * per-contact work.
+ * @param {number} nowMs Monotonic clock in milliseconds.
+ * @returns {boolean}
+ */
+export function awarenessEchoActive(nowMs) {
+  return Boolean(field) && (field.level?.(nowMs) ?? 1) > 0;
+}
+
 /**
  * How lit a contact's echo is at `nowMs`: 1 the moment a pulse front reaches
  * its distance, fading as (1 - elapsed / `fadeMs`) squared, 0 otherwise. The
@@ -99,13 +110,13 @@ export function awarenessEchoStrength(nowMs, distanceM, radiusM, pulse, echo) {
  */
 export function awarenessEchoAt(layerId, position, nowMs) {
   if (!field || !position || !layerIds.has(layerId)) return 0;
+  const level = field.level?.(nowMs) ?? 1;
+  if (!(level > 0)) return 0;
   const ground = Cesium.Ellipsoid.WGS84.scaleToGeodeticSurface(
     position,
     groundScratch,
   );
   if (!ground) return 0;
-  const level = field.level?.(nowMs) ?? 1;
-  if (!(level > 0)) return 0;
   return (
     level *
     awarenessEchoStrength(