Keep one world clock that every timed layer reads

Core told time eight ways: Date.now() in most layers, JulianDate.now() for
flights and military, Cesium's clock frozen at page load for the sky, the
weather clock for observed imagery. They now read one clock,
src/time/worldClock.js, exported as ./time.

Live is a mode, not a timestamp: while live the clock is the wall clock,
so every layer draws as before. Away from live only the layers that follow
the clock move with it. The weather products follow it, as they followed
the weather transport; a host opts others in with follow(id).

- The weather clock takes its time from the world clock and drives its
  replay, stepping its frames as before.
- Satellites, launches, flights and military (still 30 s and 15 s behind),
  transit, GNSS, Recent Imagery, street level, the HUD and the celestial
  ring read worldTimeFor(id). Cadences stay on the wall clock.
- Cesium's sky (stars, sun, moon, atmosphere, lighting) draws at the world
  time each frame instead of the moment the page loaded.
- Weather frames and satellites declare coverage() for a host's timeline.

src/celestialRing.js

@@ -1,5 +1,9 @@
 import * as Cesium from 'cesium';
 import { governorRequestRender } from './renderGovernor.js';
+import { worldClock, worldTimeFor } from './time/worldClock.js';
+
+// The ring shows the sun and moon at the time the sky is drawn.
+const skyTime = worldTimeFor('sky');
 
 /** Outer edge of the existing NVG/FLIR keyhole in normalized shader space. */
 export const KEYHOLE_OUTER_RADIUS = 1.05;
@@ -445,6 +449,12 @@ export class CelestialRing {
       if (typeof document !== 'undefined' && document.hidden) return;
       governorRequestRender('celestial-ephemeris');
     }, 60_000);
+    // A jump in the world time moves the sun and moon at once.
+    this._skyDate = new Date();
+    this._skyTime = new Cesium.JulianDate();
+    this._unsubscribeWorldTime = worldClock.subscribe(() => {
+      this._ephemerisDirty = true;
+    });
     this.setEnabled(this.enabled);
   }
 
@@ -801,7 +811,8 @@ export class CelestialRing {
       return;
     }
 
-    const time = Cesium.JulianDate.now();
+    this._skyDate.setTime(skyTime());
+    const time = Cesium.JulianDate.fromDate(this._skyDate, this._skyTime);
     if (!this._updateEphemeris(time)) return;
     this._present({ ephemerisUpdates: String(this._ephemerisUpdateCount) });
     const camera = this.viewer.camera;
@@ -890,6 +901,8 @@ export class CelestialRing {
       clearInterval(this._ephemerisTimer);
       this._ephemerisTimer = null;
     }
+    this._unsubscribeWorldTime?.();
+    this._unsubscribeWorldTime = null;
     if (this._removePostRender) this._removePostRender();
     this._removePostRender = null;
     if (this._shown()) {