Series Persistence Other Country

TODO.md cycle 78 POC C1: one series' own persistence detail for its deep-dive page's "How often has this moved?" section -- see _series_persistence()'s own docstring for the computation. Same zero-inflation reasoning as PersistentAlert.field_median_fired (POC A's own docstring): most series never fire, so a bare "fired 3 of 8" reads as notable when it may be typical for the whole stat_key -- this field is mandatory for the same reason.