A mod index can now publish three optional per-entry fields -- downloads
(total across every release), first_release and last_release (ISO days) --
which the FIND MODS listing shows in the same gold line the MODS tab uses.
The fields are additive by design: feeds that carry them stay readable by
every build that predates them (schema_version stays 1), and feeds that do
not render exactly as before. ModUpdate.statsLine builds the shared line;
the MODS tab reuses it. Parser, formatting, and parse coverage are tested.