fix(awareness): stop the subject echoing itself and cover the echo wiring

Remaining review follow-ups for the sonar pulses.

No self-echo: pulses are born 5 km out (startFraction of the radius),
but contacts nearer than that, the subject included, still lit on every
pulse birth. They now never echo. Just past the birth ring, an echo is
scaled with the front's own fade-in, so a bracket no longer flashes
before a visible front reaches it. The ring flash at the full radius is
unchanged.

Older Safari: the reduced-motion listener falls back to the media
query's addListener / removeListener when addEventListener is missing,
as other modules here already do.

Echo wiring test: a detectionHost test now paints through the real
detection lane with an echo field set, and checks the echo stroke takes
the echo colour on the default style and each style's line colour under
CRT, NVG and FLIR. CURRENT-STATE notes the 5 km no-echo zone.

src/awarenessEcho.test.mjs

@@ -45,7 +45,14 @@ test('an echo lights as a pulse front reaches the contact, then fades', () => {
 test('rim contacts fade out after the front wraps; outside the range never', () => {
   const rim = R * 0.999;
   assert.ok(strength(arrival(rim) + echo.fadeMs / 2, rim) > 0.2);
-  assert.ok(strength(pulse.periodMs * 5, 1000) > 0.99, 'inside the start ring');
+  // Inside the ring pulses are born on, nothing is ever crossed: the
+  // subject and anything within 5 km never echo, at any moment.
+  const inner = R * pulse.startFraction;
+  for (let ms = 0; ms < pulse.periodMs; ms += 10) {
+    assert.equal(strength(ms, 0), 0, 'the subject itself');
+    assert.equal(strength(ms, inner - 1), 0);
+  }
+  assert.ok(strength(pulse.periodMs * 5, inner) > 0.99, 'the birth ring');
   for (let ms = 0; ms < pulse.periodMs; ms += 50)
     assert.equal(strength(ms, R + 1), 0);
   assert.equal(strength(0, Number.NaN), 0);
@@ -74,9 +81,12 @@ test('awarenessEchoAt reads the published field, by layer, at ground level', (t)
         awarenessEchoAt('flights', contact, t0 + 300),
     ) < 0.01,
   );
-  // The field's centre is read in place, so a moved subject moves the echo.
-  Cesium.Cartesian3.clone(contact, center);
-  assert.ok(awarenessEchoAt('flights', contact, pulse.periodMs * 7) > 0.99);
+  // The field's centre is read in place, so a moved subject moves the echo:
+  // about 50 km farther, the contact lights at its new distance's moment.
+  Cesium.Cartesian3.clone(Cesium.Cartesian3.fromDegrees(3.5, 26.1, 0), center);
+  const moved = Cesium.Cartesian3.distance(center, contact);
+  assert.ok(Math.abs(moved - d) > 1000);
+  assert.ok(awarenessEchoAt('flights', contact, arrival(moved)) > 0.99);
   assert.equal(awarenessEchoAt('flights', null, t0), 0);
   clearAwarenessEchoField();
   assert.equal(getAwarenessEchoField(), null);
@@ -107,3 +117,19 @@ test('echoes scale with the field level and rest with the pulses', (t) => {
   level = 0;
   assert.equal(awarenessEchoAt('flights', contact, t0), 0);
 });
+
+test('near the subject an echo is no brighter than the fading-in front', () => {
+  const fading = { ...pulse, fadeInFraction: 0.08 };
+  const at = (distanceM) =>
+    awarenessEchoStrength(arrival(distanceM), distanceM, R, fading, echo);
+  // Born at 5 km, the front reaches full strength 8% of the way out.
+  const full = R * (fading.startFraction + 0.08 * (1 - fading.startFraction));
+  assert.equal(at(R * fading.startFraction), 0);
+  const halfway = (R * fading.startFraction + full) / 2;
+  assert.ok(Math.abs(at(halfway) - 0.5) < 0.01, `${at(halfway)}`);
+  assert.ok(at(full + 1) > 0.99);
+  assert.ok(
+    at(R * 0.999) > 0.99,
+    'the rim, and so the ring flash, is unchanged',
+  );
+});