Show the timeline only while a layer with time to show is on

The bar now needs more than the world clock being away from live: a layer
that moves with the clock must be on with time to show. In Core that is
weather radar, satellite or lightning with frames; a host adds layers by
following them. Turning the last one off hides the bar; turning it back
on brings it back.

The world clock reports this as `supported` in its state. Weather products
register their frames as coverage one by one, suspended or not, so the way
back to live stays on screen with reduced motion. Weather refreshes and
satellite catalog changes tell the clock their coverage may have changed.
A coverage that throws counts as none, and the clock reads its state once
per notification round.

src/layers/weather/clock.js

@@ -162,7 +162,34 @@ export function createWeatherClock({
         throw new Error(`Duplicate weather product: ${product.id}`);
       const entry = { product, controller: null };
       registry.set(product.id, entry);
-      releases.set(product.id, world.hold(product.id));
+      // The product moves with the world clock, and its frames are time the
+      // world clock's timeline can show while it is on, suspended or not, so
+      // the way back to live stays on screen. Both calls notify the world's
+      // listeners, which may destroy this clock: the release is in place
+      // first, and anything acquired after a destroy is let go at once.
+      let unhold = null;
+      let uncover = null;
+      const release = () => {
+        uncover?.();
+        unhold?.();
+        uncover = unhold = null;
+      };
+      releases.set(product.id, release);
+      unhold = world.hold(product.id);
+      if (!destroyed)
+        uncover = world.register({
+          id: product.id,
+          coverage: () => {
+            const marks = timesFor(product).map((time) => Date.parse(time));
+            return marks.length
+              ? { start: marks[0], end: marks.at(-1), marks }
+              : null;
+          },
+        });
+      if (destroyed) {
+        release();
+        return () => {};
+      }
       void api.refresh();
       return () => {
         if (registry.get(product.id) !== entry) return;
@@ -242,6 +269,8 @@ export function createWeatherClock({
     },
     refresh() {
       if (destroyed) return Promise.resolve(false);
+      // New frames, or products added or dropped.
+      world.coverageChanged();
       if (!canPlay()) {
         stopPlaying();
         cancelTimer();
@@ -262,7 +291,6 @@ export function createWeatherClock({
       ++generation;
       cancelTimer();
       unfollowWorld();
-      uncover();
       // The last weather clock leaves the world live, as a new one expects.
       if (driver.detach() === 0) world.goLive();
       for (const release of releases.values()) release();
@@ -292,16 +320,6 @@ export function createWeatherClock({
     else notify();
   }
   const unfollowWorld = world.subscribe(followWorld);
-  // The observed frames this clock can show, for a host's timeline.
-  const uncover = world.register({
-    id: 'weather',
-    coverage: () => {
-      const marks = getTimeline().map((time) => Date.parse(time));
-      return marks.length
-        ? { start: marks[0], end: marks.at(-1), marks }
-        : null;
-    },
-  });
   driver = world.attachDriver({
     play: () => (canPlay() ? void api.play() : false),
     pause: () => api.pause(),