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.

changelog.d/0000-timeline-one-control-and-marks.md

@@ -1,3 +1,13 @@
+- Timeline marks have labels. Hovering a tick shows its label (a launch's
+  mission name, or an earthquake's magnitude and place, such as
+  `M6.4 · 113 km SSE of Lata, Solomon Islands`), and a tap on or within 6 px
+  of a tick goes exactly to its mark rather than to the point tapped; a tap
+  elsewhere and a drag work as before. The arrow keys still step from mark to
+  mark, and the handle then tells the mark's label with the time to screen
+  readers and shows it over the track while focused. A layer's world clock
+  coverage may give each mark as `{ time, label, id, kind }` as well as a
+  bare time; `worldClock.coverage()` keeps `marks` as the sorted times and
+  adds `markDetails`, the merged marks themselves (samehkhamis, #0000).
 - The timeline marks only events: launches, and earthquakes of magnitude 6
   and up while the Earthquakes layer is on (about one every two days
   worldwide). It no longer draws a tick for every weather frame, which came