basic summaries for all pair classes, create partial classes and condense models into a single file

LightlessSync/PlayerData/Pairs/PairManager.cs

@@ -9,6 +9,9 @@ using LightlessSync.API.Dto.User;
 
 namespace LightlessSync.PlayerData.Pairs;
 
+/// <summary>
+///     in memory state for pairs, groups, and syncshells
+/// </summary>
 public sealed class PairManager
 {
     private readonly object _gate = new();