A row plays only with a timeline to play; the speed shown follows the world clock; mark labels merge whole and cut by character

A weather row's play started the world clock even with no time to show,
where there is no timeline; it now does nothing then, as the timeline's
own Play cannot be reached. A weather layer made without a weather clock
does nothing once destroyed, so enabling it after builds no clock.

The timeline's speed follows the world clock's rate whoever set it, and
its Play runs at that rate, so it and a row's play never run at different
rates; a rate not offered shows as the nearest offered speed. Choosing a
speed sets the clock's rate first, also while a driver steps.

Marks merge on their whole label and are cut for showing after, so two
whose labels only differ past the cut stay two, and the cut counts
graphemes (code points where the runtime cannot tell them), so an emoji is
never split.

Tests that compared a DOM element with null built an assertion message
from the element when they failed, which hung the run; they compare plain
values now, so they fail at once. The timeline tests put the clock back to
real time after each.

docs/CURRENT-STATE.md

@@ -8545,8 +8545,10 @@ 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 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) of up to 60 characters;
-a longer one ends in `…`, and a label is never wider than the track. A zoom or
+index shows names; an earthquake's magnitude and place). Marks merge on their
+whole label; one longer than 60 characters (graphemes, so an emoji is never
+split) then shows cut, ending in `…`, and a label is never wider than the
+track. A zoom or
 a pan clears a hovered label. 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
@@ -8579,12 +8581,15 @@ The catalog exposes `weatherClock` and destroys it with its lifetime signal. It
 has no time controls of its own: `weatherClock.world` is the world clock it
 follows, where time moves. A weather row's `play`, `latest` and `step` params
 move that world clock as the timeline does: `play` runs at the speed chosen on
-the timeline (the world clock keeps its rate while paused and live), from the
-start of the covered time when live or at its end, and does nothing while
-every product the row shows is suspended. A weather layer built without a
+the timeline (the world clock keeps its rate while paused and live, and the
+timeline's speed selector shows the clock's rate, or the nearest offered speed
+for a rate a host set), from the start of the covered time when live or at its
+end, and does nothing while every product the row shows is suspended or while
+there is no covered time. A weather layer built without a
 shared clock builds one of its own on the page's world clock when first
 enabled (`createWeatherClock({ shared: false })`) and keeps it for its
-lifetime; destroying it leaves the world's time where it is, and only the last
+lifetime; destroying it leaves the world's time where it is, and enabling it
+after that does nothing, and only the last
 shared weather clock destroyed sends the world live. Neither has a frame replay
 of its own.