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.js

@@ -13,7 +13,7 @@ import * as Cesium from 'cesium';
  * @property {Cesium.Cartesian3} center Subject's ground point. The publisher
  *   may move it in place; it is read on every query.
  * @property {number} radiusM Range the pulses grow to.
- * @property {{count: number, periodMs: number, startFraction: number}} pulse
+ * @property {{count: number, periodMs: number, startFraction: number, fadeInFraction?: number}} pulse
  * @property {{fadeMs: number, layerIds: string[]}} echo
  * @property {(nowMs: number) => number} [level] How far the pulses are
  *   switched on, 0 to 1; echoes scale with it, so they rest with the pulses.
@@ -50,22 +50,27 @@ export function getAwarenessEchoField() {
  * its distance, fading as (1 - elapsed / `fadeMs`) squared, 0 otherwise. The
  * strongest pulse wins. A front that has wrapped back to the centre still
  * counts for the contact it last crossed, so rim contacts fade out instead of
- * cutting off.
+ * cutting off. Nothing nearer than the birth ring `startFraction` out echoes,
+ * and within the pulse's `fadeInFraction` the echo is scaled down with it.
  * @param {number} nowMs Monotonic clock in milliseconds.
  * @param {number} distanceM Contact's distance from the subject.
  * @param {number} radiusM Range the pulses grow to.
- * @param {{count: number, periodMs: number, startFraction: number}} pulse
+ * @param {{count: number, periodMs: number, startFraction: number, fadeInFraction?: number}} pulse
  * @param {{fadeMs: number}} echo
  * @returns {number} 0 to 1.
  */
 export function awarenessEchoStrength(nowMs, distanceM, radiusM, pulse, echo) {
-  if (!Number.isFinite(distanceM) || distanceM < 0 || distanceM > radiusM)
+  // Pulses are born `startFraction` out, so nothing nearer (the subject
+  // itself, its wingmen) is ever crossed; without this it lit on every birth.
+  if (
+    !Number.isFinite(distanceM) ||
+    distanceM < radiusM * pulse.startFraction ||
+    distanceM > radiusM
+  )
     return 0;
   // The phase at which a pulse front sits at this distance.
-  const touchPhase = Math.max(
-    0,
-    (distanceM / radiusM - pulse.startFraction) / (1 - pulse.startFraction),
-  );
+  const touchPhase =
+    (distanceM / radiusM - pulse.startFraction) / (1 - pulse.startFraction);
   let strength = 0;
   for (let index = 0; index < pulse.count; index++) {
     const cycle = nowMs / pulse.periodMs + index / pulse.count;
@@ -77,7 +82,10 @@ export function awarenessEchoStrength(nowMs, distanceM, radiusM, pulse, echo) {
     if (elapsedMs < echo.fadeMs)
       strength = Math.max(strength, (1 - elapsedMs / echo.fadeMs) ** 2);
   }
-  return strength;
+  // A front still fading in over `fadeInFraction` of its travel echoes no
+  // brighter than it shows.
+  const fadeIn = pulse.fadeInFraction ?? 0;
+  return fadeIn > 0 ? strength * Math.min(1, touchPhase / fadeIn) : strength;
 }
 
 /**