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.

src/cockpitSignalFocus.test.mjs

@@ -209,7 +209,7 @@ test('rows render the signals the controller presents', async (t) => {
   );
   assert.equal(status.className, 'warning');
   assert.equal(status.querySelector('div > strong').textContent, 'UNKNOWN');
-  assert.equal(status.querySelector('button'), null);
+  assert.equal(status.querySelector('button') === null, true);
 });
 
 test('actual Context refresh retains the focused signal button across unchanged and changed details', async (t) => {
@@ -315,7 +315,7 @@ test('removing the focused identity uses the shown briefing tab once and permits
   assert.equal(document.activeElement === f.signalTab(), true);
   assert.equal(f.focusCalls(), calls + 1);
   assert.equal(f.losses(), 0);
-  assert.equal(f.signalTab().closest('[hidden]'), null);
+  assert.equal(f.signalTab().closest('[hidden]') === null, true);
   // Traversal carries on forward from the footer tab.
   f.tab();
   assert.equal(document.activeElement === f.pageTab(1), true);