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.test.mjs

@@ -4,6 +4,7 @@ import test from 'node:test';
 import assert from 'node:assert/strict';
 import * as Cesium from 'cesium';
 import {
+  awarenessEchoActive,
   awarenessEchoAt,
   awarenessEchoStrength,
   clearAwarenessEchoField,
@@ -114,8 +115,13 @@ test('echoes scale with the field level and rest with the pulses', (t) => {
   assert.ok(
     Math.abs(awarenessEchoAt('flights', contact, t0) - full / 2) < 1e-9,
   );
+  assert.equal(awarenessEchoActive(t0), true);
   level = 0;
   assert.equal(awarenessEchoAt('flights', contact, t0), 0);
+  // At rest the overlay skips echoes for the whole frame.
+  assert.equal(awarenessEchoActive(t0), false);
+  clearAwarenessEchoField();
+  assert.equal(awarenessEchoActive(t0), false);
 });
 
 test('near the subject an echo is no brighter than the fading-in front', () => {