feat(awareness): add calm sonar pulses that flash the range ring and echo contacts

Pulses: three dim rings grow from the Global Context subject to the
250 km range ring at constant speed, one every 3 s, each taking 9 s.
Each fades in over its first 8% of travel, then fades as
(1 - travel)^1.5 to about a third of its 60% peak, so it arrives at the
ring dim but visible. They are 3 px in the ring's colours, as polylines
with CallbackProperty positions and colours that share the ring's
centre object, so they follow a moving subject.

Ring: while pulses run, the 250 km ring rests unseen and flashes to full
strength as each pulse reaches it, fading over 1.8 s. Without pulses
(reduced motion, or a scene with no primitives or preRender hook) it
stays on screen as before.

Gradient: behind each pulse a radial band in the same blue fades inward
over 35% of the radius. One flat, translucent, unpickable 250 km disc
draws it with a small Fabric material. A preRender listener writes the
pulse radii and opacities into two vec4 uniforms each frame. The disc
(0.25 degree granularity, ~3.5 ms to build) is rebuilt only after the
subject moves 2 km.

Echoes: as a pulse front crosses a flight, military aircraft or AIS
vessel, the detection overlay draws a second bracket in the pulse blue
around that contact's own bracket, spreading outward as it fades over
1.4 s. Awareness publishes the field (centre, radius, pulse timing,
echo styling) through the new src/awarenessEcho.js. The overlay reads
it, so no layer's icon or colour is touched. Mapped installations have
no detection bracket, so they do not echo.

Rendering cost: pulses never hold continuous rendering. They ask for a
frame every 80 ms, the Cyber sonar's parked cadence, so a parked
Contacts view repaints at ~12.5 fps while a subject is shown. This
changes the "parked Contacts stays quiet" behaviour on purpose.
clearVisual stops the timer and removes the disc, the listener and the
echo field.

awarenessRangeRingPositions can now reuse a result array, and passes the
WGS84 ellipsoid explicitly so a reused Cartesian never lands in the
ellipsoid argument. Tuning lives in AWARENESS_RANGE_PULSE,
AWARENESS_RANGE_ECHO and AWARENESS_RANGE_RING_STYLE. src/awarenessEcho.js
is registered in the awareness-layer, application-components and
application-layer-construction packages.

Tests cover:
- the pulse curve and calm rate;
- the ring flash once per arrival, and the static ring without pulses;
- the 80 ms frame cadence and its stop;
- the partial-scene fallback;
- the gradient disc: unpickable, uniform values, rebuild and cleanup;
- per-frame evaluation, and the echo field following the subject;
- awarenessEcho strength, rim wrap, layer filter and altitude handling;
- the detection renderer's echo-bracket band and outward spread.

src/awarenessEcho.test.mjs

@@ -0,0 +1,84 @@
+// src/awarenessEcho.test.mjs — the Global Context echo field: when a sonar
+// pulse front crosses a flight or vessel, its bracket echo lights and fades.
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import {
+  awarenessEchoAt,
+  awarenessEchoStrength,
+  clearAwarenessEchoField,
+  getAwarenessEchoField,
+  setAwarenessEchoField,
+} from './awarenessEcho.js';
+
+const R = 250_000;
+const pulse = { count: 3, periodMs: 9000, startFraction: 0.02 };
+const echo = { fadeMs: 1400, layerIds: ['flights', 'ais-live-vessels'] };
+const strength = (nowMs, distanceM) =>
+  awarenessEchoStrength(nowMs, distanceM, R, pulse, echo);
+/** The moment pulse 0's front sits at `distanceM`, in a later cycle. */
+const arrival = (distanceM) =>
+  ((distanceM / R - pulse.startFraction) / (1 - pulse.startFraction)) *
+    pulse.periodMs +
+  pulse.periodMs * 4;
+
+test('an echo lights as a pulse front reaches the contact, then fades', () => {
+  const d = 100_000;
+  const t0 = arrival(d);
+  assert.ok(Math.abs(strength(t0, d) - 1) < 1e-6);
+  assert.ok(Math.abs(strength(t0 + echo.fadeMs / 2, d) - 0.25) < 1e-6);
+  assert.equal(strength(t0 + echo.fadeMs + 1, d), 0);
+  assert.equal(strength(t0 - 50, d), 0, 'dark just before the front');
+  // Every pulse lights it once a period: `count` separate flashes in any
+  // one-period window that starts after a flash has faded.
+  let flashes = 0;
+  let wasLit = false;
+  const from = t0 + echo.fadeMs + 1;
+  for (let ms = 0; ms < pulse.periodMs; ms += 10) {
+    const lit = strength(from + ms, d) > 0.5;
+    if (lit && !wasLit) flashes++;
+    wasLit = lit;
+  }
+  assert.equal(flashes, pulse.count);
+});
+
+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');
+  for (let ms = 0; ms < pulse.periodMs; ms += 50)
+    assert.equal(strength(ms, R + 1), 0);
+  assert.equal(strength(0, Number.NaN), 0);
+  assert.equal(strength(0, -1), 0);
+});
+
+test('awarenessEchoAt reads the published field, by layer, at ground level', (t) => {
+  t.after(clearAwarenessEchoField);
+  const contact = Cesium.Cartesian3.fromDegrees(3.5, 27, 0);
+  assert.equal(awarenessEchoAt('flights', contact, 0), 0, 'no field, no echo');
+  const center = Cesium.Cartesian3.fromDegrees(3, 27, 0);
+  setAwarenessEchoField({ center, radiusM: R, pulse, echo });
+  assert.ok(getAwarenessEchoField());
+  const d = Cesium.Cartesian3.distance(center, contact);
+  const t0 = arrival(d);
+  assert.ok(awarenessEchoAt('flights', contact, t0) > 0.99);
+  assert.ok(awarenessEchoAt('ais-live-vessels', contact, t0) > 0.99);
+  // Layers outside the echo list never light.
+  assert.equal(awarenessEchoAt('cctv', contact, t0), 0);
+  assert.equal(awarenessEchoAt('military-installations', contact, t0), 0);
+  // An aircraft 11 km up echoes with the ground point under it.
+  const aloft = Cesium.Cartesian3.fromDegrees(3.5, 27, 11_000);
+  assert.ok(
+    Math.abs(
+      awarenessEchoAt('flights', aloft, t0 + 300) -
+        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);
+  assert.equal(awarenessEchoAt('flights', null, t0), 0);
+  clearAwarenessEchoField();
+  assert.equal(getAwarenessEchoField(), null);
+  assert.equal(awarenessEchoAt('flights', contact, t0), 0);
+});