Timeline marks carry labels; a tap by a tick goes to its mark

A layer's world clock coverage may now give each mark as an object,
{ time, label?, id?, kind? }, as well as a bare epoch ms number, and the
two may be mixed. worldClock.coverage() keeps marks as the sorted list of
distinct times, so existing readers are unchanged, and adds markDetails:
the merged marks in time order (then kind, label and id), each frozen and
holding only the fields given. Marks given alike by several sources appear
once, a bare time is dropped where a labelled mark shares it, and a mark
with no finite time is ignored. The contract is documented in the
worldClock JSDoc and docs/APPLICATION.md.

Launches label their marks with the mission's name (the launch's where it
has none), cut to 27 characters in capitals as the mission index shows
names. Earthquakes of magnitude 6 and up label theirs with the magnitude
as the map shows it and the feed's place cut to 40 characters, such as
"M6.4 · 113 km SSE of Lata, Solomon Islands"; the layer now tells the clock
when the labels change as well as the times.

On the timeline, a pointer within 6 px of a tick shows its label over the
track (nothing for an unlabelled mark), and a tap there goes exactly to
the mark's time instead of the point tapped. A tap elsewhere and a drag
work as before, and ticks stay ticks. The arrow keys still step from mark
to mark; with the handle on a mark, its labels follow the time in the
handle's aria-valuetext, and show over the track while it has focus.

src/data/rocketLaunches.test.mjs

@@ -1244,7 +1244,15 @@ test('real mission build, select, refresh, deselect, disable, and destroy paths
     assert.equal(worldClock.coverage(), null, 'nothing loaded, no time to show');
     await rocketLaunchesLayer.update();
     const at = Date.parse(launchTime);
-    assert.deepEqual(worldClock.coverage(), { start: at, end: at, marks: [at] });
+    // The launch is a mark labelled with its mission's name.
+    assert.deepEqual(worldClock.coverage(), {
+      start: at,
+      end: at,
+      marks: [at],
+      markDetails: [
+        { time: at, label: 'GAUNTLET PAYLOAD', id: 'mission-runtime', kind: 'launch' },
+      ],
+    });
     assert.equal(worldClock.isSupported(), true, 'loaded launches bring up the timeline');
 
     const entities = dataSources[0].entities.values;