Add support for client-side replays (#17168)

Content.Client/Replay/Spectator/ReplaySpectatorSystem.cs

@@ -6,6 +6,8 @@ using Robust.Client.Player;
 using Robust.Client.Replays.Playback;
 using Robust.Client.State;
 using Robust.Shared.Console;
+using Robust.Shared.Network;
+using Robust.Shared.Serialization.Markdown.Mapping;
 
 namespace Content.Client.Replay.Spectator;
 
@@ -29,7 +31,7 @@ public sealed partial class ReplaySpectatorSystem : EntitySystem
     [Dependency] private readonly SharedContentEyeSystem _eye = default!;
     [Dependency] private readonly IReplayPlaybackManager _replayPlayback = default!;
 
-    private SpectatorPosition? _oldPosition;
+    private SpectatorData? _spectatorData;
     public const string SpectateCmd = "replay_spectate";
 
     public override void Initialize()
@@ -58,15 +60,19 @@ public sealed partial class ReplaySpectatorSystem : EntitySystem
         _replayPlayback.ReplayPlaybackStopped -= OnPlaybackStopped;
     }
 
-    private void OnPlaybackStarted()
+    private void OnPlaybackStarted(MappingDataNode yamlMappingNode, List<object> objects)
     {
         InitializeMovement();
-        SetSpectatorPosition(default);
         _conHost.RegisterCommand(SpectateCmd,
             Loc.GetString("cmd-replay-spectate-desc"),
             Loc.GetString("cmd-replay-spectate-help"),
             SpectateCommand,
             SpectateCompletions);
+
+        if (_replayPlayback.TryGetRecorderEntity(out var recorder))
+            SpectateEntity(recorder.Value);
+        else
+            SetSpectatorPosition(default);
     }
 
     private void OnPlaybackStopped()