Weather rows play at the timeline's speed; the frame time shows under reduced motion; tidy the timeline's mark label

A weather row's play ran the world clock at whatever rate it had, and
going live set that back to 1, so the speed chosen on the timeline was
lost. The world clock now keeps its rate while paused and live, and the
timeline's speed sets it whenever it is chosen, so any play runs at it.
A row's play starts from the start of the covered time when live or at
its end, as the timeline's does, and does nothing while every product the
row shows is suspended, as its reduced-motion note says.

Live, the Weather panel's frame time is the newest of each product's
frames, suspended or not, so it no longer goes away under reduced motion,
on a hidden page or without an imagery host.

A weather layer made without a weather clock builds its own when first
enabled rather than when made, so a layer never turned on leaves the
world clock alone, and its own clock (createWeatherClock({ shared: false }))
leaves the world's time where it is when destroyed; only the last shared
weather clock sends the world live.

On the timeline, a zoom or a pan clears the hovered tick's label, a label
is never wider than the track and ends in an ellipsis, and a mark's label
is cut to 60 characters (WORLD_CLOCK_MARK_LABEL_LENGTH).

The changelog entry says what a row's step does, names the earthquake
layer's new exports and says it registers with the world clock.

docs/APPLICATION.md

@@ -199,8 +199,8 @@ the unregister; call `worldClock.coverageChanged()` when that time changes.
 `{ start, end, marks? }` in epoch ms. `marks` are the moments worth going to,
 drawn as ticks the timeline's arrow keys step to and a tap on or within 6 px of
 snaps to. Each mark is a number, or `{ time, label?, id?, kind? }`: `label` is
-short text shown when the pointer is on the tick and told with the time while
-the handle is on the mark, `id` the layer's own key for the event and `kind`
+short text (cut to 60 characters, ending in `…`) shown when the pointer is on
+the tick and told with the time while the handle is on the mark, `id` the layer's own key for the event and `kind`
 what sort of event it is (`'launch'`, `'earthquake'`). The two forms may be
 mixed; a mark without a finite time, and a `label`, `id` or `kind` that is not
 a non-empty string, are ignored. `worldClock.coverage()` returns the union of