Keep the hour it is now on the axis

The visible range is drawn from the events in view, and in the evening the
events are behind you: the bounds end at six, everything after that is in the
trailing strip, and the now line has nowhere to land. The app stops saying
where in the day you are, which is most of what it is open for. Same at seven
in the morning, from the other end.

So when today is one of the columns, the axis takes that one hour in, and
everything the widening reached over folds behind it. At half eleven at night
the fixture week reads 7am to 8pm as usual, then an "8pm to 11pm" strip, then
one 11pm row with the line in it. A row and a strip, not four rows of empty
evening. Only the hour itself is opened, so how far the clock has drifted past
the last event costs nothing.

The pin sits on top of the hysteresis rather than inside it. previous.current
still holds what the events and the user asked for, so an hour pinned open
tonight cannot accumulate into the bounds the grid remembers tomorrow.

One consequence, and it is the only place the axis is allowed to move under
you: page to a week that does not contain today and the row goes again, along
with the strip it was sitting under.

The tick that drives it is an hour long, not a minute, because that is how
often the answer changes. It lives in a new useClock alongside the minute tick
the now line already had, which moves there out of GridNowLine. Both schedule
off the clock rather than off an interval, so a machine that was asleep
catches up on the next tick instead of drifting further out every hour.

The browser suite has to say what time it is now. The shape of the axis
depends on the hour a run happens at, so anything measuring a row height or
counting strips pins the clock to the middle of the working day, the same way
it already pins the theme and the week start. The three tests that are about
the clock ask for half eleven at night, which the fixture leaves empty.

docs/design.md

@@ -30,6 +30,14 @@ The range expands immediately when it needs to, but only contracts when it would
 hours or more. Without that hysteresis the axis flickers as you page through weeks, and a
 flickering axis destroys the positional memory that makes a keyboard-driven calendar fast.
 
+The hour you are in is the one exception to the range being the events' business. At half
+eleven at night the events stopped hours ago, so the now line has nowhere to land and the app
+stops telling you where in the day you are, which is most of what a calendar is open for. So
+when today is on screen the axis takes that hour in, and folds away everything it reached over
+to get there: the axis grows by a row and a strip, not by an evening. Page to a week that does
+not contain today and it goes again, which is the one place the axis is allowed to move under
+you.
+
 Interior gaps stay at full scale. A three-hour hole on a Wednesday afternoon is the most
 useful thing on the screen, because it is where work goes, and folding it automatically would
 make a packed day look identical to an open one. But you can fold one deliberately with `z`,