Inputs to createTimelineGrouping. Every reactive knob is a
zero-arg accessor, so a Signal (or an input()) can be passed
directly and a constant is just () => 'day'.
Timestamp accessor. Not a signal itself, but it is called inside the
groups computation, so a caller that closes over one - as the organism
does over its [dateAccessor] input - has those reads tracked like any
other and re-groups when the accessor is rebound.