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.

docs/CURRENT-STATE.md

@@ -1785,7 +1785,10 @@ destroying the layer cancels pending work and ignores late results. While on,
 the layer registers its quakes of magnitude 6 and up
 (`EARTHQUAKE_MARK_MAGNITUDE`) with the world clock as timeline marks, spanning
 only their own times, and none while off; the map still draws every M2.5+
-quake.
+quake. Each mark is labelled with the magnitude as the map shows it and the
+feed's place, cut to 40 characters (`earthquakeMarkLabel` in
+`src/layers/earthquakes/model.js`: `M6.4 · 113 km SSE of Lata, Solomon
+Islands`, or `M6.4` with no place).
 
 ## Rendered-mesh floor readings — November 9, 2026
 
@@ -8540,7 +8543,14 @@ clock moves all enabled observation rows with the world clock; the world
 timeline is the only time control. Each product's advertised times span time on
 that timeline but are not marks on it: the timeline marks only events (launches,
 and earthquakes of magnitude 6 and up while that layer is on), so its arrow keys
-step to those, or along the track where there are none. A weather row's `step`
+step to those, or along the track where there are none. A mark may carry a
+label (a launch's mission name, cut to 27 characters in capitals as the mission
+index shows names; an earthquake's magnitude and place). The pointer within
+6 px of a tick shows its label over the track, and a tap there goes to the
+mark's time exactly; a tap elsewhere goes to the time under it and a drag
+scrubs. With the handle on a mark, its label follows the time in the handle's
+`aria-valuetext` and shows over the track while the handle has focus. The
+coverage contract is in [APPLICATION.md](./APPLICATION.md). A weather row's `step`
 param goes to the previous or next weather frame. Each product selects its
 nearest frame at or before the world time, never a future frame or an
 interpolation.