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/PlayerRig.cs

@@ -15,6 +15,9 @@ public partial class PlayerRig : Node
     public const float MoveAcceleration = 8f;      // m/s²
     public const float StickDeadzone = 0.2f;
     public static bool SmoothTurn = false;
+    // Tracking timed by the render clock (bible v2: uniform motion, constant ~2-tick lag) vs by
+    // arrival (less lag, 18 Hz speed beat). Menu toggle so both can be compared in the headset.
+    public static bool SteadyTracking = true;
     public const float SnapTurnDegrees = 30f;
     public const float SmoothTurnSpeed = 2.5f;     // rad/s
     public const float SnapTurnThreshold = 0.7f;
@@ -144,7 +147,8 @@ public partial class PlayerRig : Node
         _driver.RebuildRequested |= _rebuild;
         _rebuild = false;
         Transform3D o = _origin.GlobalTransform;
-        _driver.Tick(profile, ref o, _camera.Transform, _left.Transform, _right.Transform, _moveVelocity, dt);
+        _driver.Tick(profile, ref o, _camera.Transform, _left.Transform, _right.Transform, _moveVelocity, dt,
+            SteadyTracking ? TrackingAge() : null);
         _origin.GlobalTransform = o;
         _lastOrigin = _origin.GlobalTransform;
         _haveLastOrigin = true;
@@ -153,6 +157,24 @@ public partial class PlayerRig : Node
         _interpolateOrigin = true;
     }
 
+    // The tracking poses read in this tick were delivered with the last frame, predicted for its render
+    // time. In physics ticks: render time of that frame ≈ (physics frames + fraction) at its _process,
+    // plus one frame delta for the frame these ticks belong to (bible v2 recipe).
+    private double TrackingAge()
+    {
+        float rate = Engine.PhysicsTicksPerSecond;
+        ulong frame = Engine.GetProcessFrames();
+        if (frame != _sampleFrame)
+        {
+            _sampleFrame = frame;
+            _sampleTicks = _renderTicks + _renderDelta * rate;
+        }
+        return (Engine.GetPhysicsFrames() - _sampleTicks) / rate;
+    }
+
+    private double _renderTicks, _renderDelta, _sampleTicks;
+    private ulong _sampleFrame = ulong.MaxValue;
+
     // The origin on screen is interpolated; physics must run on the real one. Anyone else who moved
     // the origin since (teleport marker, menu) is respected as the new real one.
     private void RestorePhysicsOrigin()
@@ -168,6 +190,8 @@ public partial class PlayerRig : Node
 
     public override void _Process(double delta)
     {
+        _renderTicks = Engine.GetPhysicsFrames() + Engine.GetPhysicsInterpolationFraction();
+        _renderDelta = delta;
         if (_interpolateOrigin && !Spectator)
         {
             Transform3D current = _origin.GlobalTransform;