Rank/skill snapshot for a player in a single match. Tier/Division are the
values after the match; Prev* are the values from before it. Mu/Sigma
are the underlying TrueSkill values (display MMR is Mu * 20 + 100).
Current skill snapshot for a single playlist from Skills/GetPlayersSkills.
Mu/Sigma/Tier/Division mirror the match-history values, but this also
reports cumulative counters (WinStreak, MatchesPlayed,
PlacementMatchesPlayed) and PsyNet’s own MMR field. These are
point-in-time as of the request, not tied to any particular match.