The timeline marks launches and strong earthquakes, not every weather frame

The timeline drew a tick for every registered mark, and the weather clock
registered every radar, satellite and lightning frame, which come every few
minutes, so the track was a wall of dashes and the arrow keys crept from
frame to frame.

Weather products now register only the span of their frames (start and
end, as before) with no marks. Holds, the gate and the next-frame wait
while the world plays are unchanged. A weather row's step param goes to
the previous or next weather frame (every shown product's frames, and the
row's own while it is suspended) rather than to the timeline's marks.

The Earthquakes layer registers with the world clock once, at init, and
unregisters on destroy. While on, its quakes of magnitude 6 and up
(EARTHQUAKE_MARK_MAGNITUDE) are marks, and its coverage is their own span,
or none without any, so it never stretches the timeline beyond the feed.
Off, it has none. It tells the clock when the set of marks changes. The
map still draws every M2.5+ quake.

Launches stay marks. With no mark ahead or behind, the arrow keys step
along the track as shown.

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

@@ -1,3 +1,13 @@
+- 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
+  every few minutes and made the track a wall of dashes; the weather frames
+  still set the time the timeline covers, and Play still waits at each next
+  frame until it loads. With no mark ahead, the arrow keys step along the
+  track. The map still draws every earthquake, and an earthquake layer never
+  widens the timeline beyond its strong quakes' own times. A weather row's
+  `step` param goes to the previous or next weather frame (samehkhamis,
+  #0000).
 - The main timeline is the only time control. The Weather panel no longer
   has a slider, step buttons, Latest or Play of its own; it shows the frame
   time instead: the newest frame and its age while live, the timeline time