Find a reference time for computing pulse_index

See https://github.com/scipp/essreduce/pull/180#discussion_r1949052455 for context.

When doing pulse-skipping, We need a reliable way to define a reference start time of the run, so we can determine to which pulse in a stride the events are coming from.
The reference time needs to be fixed, in case the data comes in in chunks (cannot simply use the min event_time_offset for each new piece of data).
Some pulses might have no events, so pulses are not always consecutive.

Using a global reference like epoch may work, but it seems difficult to know only from that if the filewriter started recording in the middle of a frame, or at the start of a frame.

It seems the only way would be to look at the open/close times of the pulse-skipping chopper and determine pulse indices from that.
This sounds complicated as we would need to flag which is the pulse-skipping chopper, propagate the open-close times back to the source, and assign pulse indices. The open-close times would need to be in the global timing reference, probably using the chopper TDC time stamps...

Start with the linked pull request discussion and trace the pulse-skipping logic that computes pulse_index. Determine how a fixed run reference can remain consistent across data chunks and missing pulses, then verify that pulse indices can be assigned using the available chopper timing information.