Render-clock tracking timestamps (menu toggle), physics step cap 6, docs for step 2.7

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/MainMenu.cs

@@ -20,6 +20,7 @@ public partial class MainMenu : PanelContainer
     public event Action Clicked;
     public event Action<bool> SpectatorToggled;
     public event Action<bool> SmoothTurnToggled;
+    public event Action<bool> LowLagTrackingToggled;
     public event Action<float> StrengthChanged;
 
     public TabContainer Tabs { get; private set; }
@@ -155,6 +156,10 @@ public partial class MainMenu : PanelContainer
         SpectatorButton = MakeToggle("Spectator: off", "Spectator: on (grab-fly)", on => SpectatorToggled?.Invoke(on));
         modeRow.AddChild(SpectatorButton);
         player.AddChild(modeRow);
+        // Compare in the headset: tracking timed by the render clock (even motion) or by arrival (less lag).
+        HBoxContainer trackingRow = new();
+        trackingRow.AddChild(MakeToggle("Tracking: steady", "Tracking: low lag", on => LowLagTrackingToggled?.Invoke(on)));
+        player.AddChild(trackingRow);
         Tabs.AddChild(player);
 
         // World