fix(runtime/camera) #429: presented player and chase camera share the object clock

Two halves of the felt run-hitch (the visible one-frame player lurch):

- The presentation lerp normalized the pending object-clock time by the
  fixed 30 Hz MinQuantum, but retail's object clock simulates
  VARIABLE-length quanta (CPhysicsObj::update_object 0x00515D10: capped
  at MaxQuantum, everything above MinQuantum runs as ONE step). After a
  long frame the view froze for the quantum and then fast-replayed it.
  ComputeRenderPosition now spans the ACTUAL last quantum
  (_lastQuantumSeconds), and PresentedDeltaSeconds accounts continuous
  presented time across quantum boundaries.

- The chase camera damped toward the presented player using wall dt
  while the player presents on the object clock, so a long frame
  stepped the camera far past the under-advanced player — measured up
  to ~1 m of camera/player decoherence in a single frame. Retail ties
  camera update to the physics-update callback
  (SmartBox::PlayerPhysicsUpdatedCallback 0x00452d60), i.e. the same
  clock as the body; both chase cameras now integrate
  PresentedDeltaSeconds. Manual zoom/pitch adjustment stays on wall dt
  (a user-input rate, not target chasing).

Owner gate: camera-vs-player boom-length change fell from ~1 m spikes
to 0.2-1.2 cm median on long frames; teleports settle clean. Two
Runtime tests updated to pin the continuous-rate contract. The
temporary PlayerPresentationProbe apparatus that measured this is
retired with the fix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/CameraFrameController.cs

@@ -94,11 +94,23 @@ internal sealed class CameraFrameController : ICameraFramePhase
             _spatialReconciler.Reconcile();
 
         MovementResult result = playerFrame.Movement;
+        // #429 defect 2: the chase camera smooths toward the PRESENTED player
+        // position, which lives on the retail 30 Hz object clock (see
+        // PlayerMovementController.PresentedDeltaSeconds). Integrating the
+        // damping with wall dt made the camera step full wall time on long
+        // frames while the presented player under-advanced against the
+        // quantum — measured ~1 m of camera/player decoherence in one frame,
+        // the felt run-hitch. Retail ties the camera to the physics-update
+        // callback (SmartBox::PlayerPhysicsUpdatedCallback 0x00452d60), i.e.
+        // the same clock as the body; the presented delta restores that.
+        // Manual zoom/pitch adjustment above stays on wall dt — it is a
+        // user-input rate, not target chasing.
+        float cameraDt = controller.PresentedDeltaSeconds;
         legacy.Update(
             result.RenderPosition,
             controller.Yaw,
             isOnGround: result.IsOnGround,
-            dt: timing.SimulationDeltaSecondsSingle);
+            dt: cameraDt);
 
         retail?.Update(
             result.RenderPosition,
@@ -106,7 +118,7 @@ internal sealed class CameraFrameController : ICameraFramePhase
             playerVelocity: controller.BodyVelocity,
             isOnGround: result.IsOnGround,
             contactPlaneNormal: controller.ContactPlane.Normal,
-            dt: timing.SimulationDeltaSecondsSingle,
+            dt: cameraDt,
             cellId: controller.CellId,
             selfEntityId: controller.LocalEntityId,
             trackedTargetPoint: _combatTarget.GetTrackedTargetPoint());

src/AcDream.App/Rendering/PlayerPresentationProbe.cs

@@ -1,75 +0,0 @@
-using System.Diagnostics;
-using System.Globalization;
-using System.Numerics;
-
-namespace AcDream.App.Rendering;
-
-/// <summary>
-/// TEMPORARY #429 apparatus. One CSV row per rendered frame:
-/// <c>seconds,camX,camY,camZ,playerX,playerY,playerZ</c>, where seconds is a
-/// Stopwatch-derived monotonic time and player is the frame's
-/// <c>PlayerViewPosition</c> — the presented local-player position driving
-/// lighting/visibility this frame. Off unless
-/// <c>ACDREAM_PROBE_PLAYER_PRESENT=&lt;path&gt;</c> is set (one null check per
-/// frame). Analysis: a hitch frame shows the player's per-frame delta
-/// collapsing to ~0 or doubling; whether the SAME frame's dt is smooth
-/// separates a presentation-phase bug (pack frame graph sampling a stale
-/// snapshot) from genuine frame-pacing spikes. Strip with the #429 fix.
-/// </summary>
-internal sealed class PlayerPresentationProbe : IDisposable
-{
-    private readonly StreamWriter _writer;
-    private readonly long _startTimestamp = Stopwatch.GetTimestamp();
-    private int _linesSinceFlush;
-
-    private PlayerPresentationProbe(StreamWriter writer)
-    {
-        _writer = writer;
-        _writer.WriteLine("seconds,camX,camY,camZ,playerX,playerY,playerZ");
-    }
-
-    internal static PlayerPresentationProbe? CreateFromEnvironment()
-    {
-        string? path = Environment.GetEnvironmentVariable("ACDREAM_PROBE_PLAYER_PRESENT");
-        if (string.IsNullOrWhiteSpace(path))
-            return null;
-
-        try
-        {
-            return new PlayerPresentationProbe(new StreamWriter(path, append: false));
-        }
-        catch (Exception ex)
-        {
-            Console.Error.WriteLine(
-                $"[player-present] probe file '{path}' could not be opened: {ex.Message}");
-            return null;
-        }
-    }
-
-    internal void Observe(in Vector3 cameraPosition, in Vector3 playerViewPosition)
-    {
-        double seconds = (Stopwatch.GetTimestamp() - _startTimestamp)
-            / (double)Stopwatch.Frequency;
-        _writer.WriteLine(string.Create(
-            CultureInfo.InvariantCulture,
-            $"{seconds:F6},{cameraPosition.X:F4},{cameraPosition.Y:F4},{cameraPosition.Z:F4},{playerViewPosition.X:F4},{playerViewPosition.Y:F4},{playerViewPosition.Z:F4}"));
-        if (++_linesSinceFlush >= 240)
-        {
-            _linesSinceFlush = 0;
-            _writer.Flush();
-        }
-    }
-
-    public void Dispose()
-    {
-        try
-        {
-            _writer.Flush();
-            _writer.Dispose();
-        }
-        catch
-        {
-            // A probe must never turn teardown fallible.
-        }
-    }
-}

src/AcDream.App/Rendering/WorldRenderFrameBuilder.cs

@@ -450,10 +450,6 @@ internal sealed class RuntimeWorldFrameEnvironmentPreparation
     private readonly HashSet<uint> _visibleCells = [];
     private bool _visibleCellsValid;
 
-    /// <summary>TEMPORARY #429 — see <see cref="PlayerPresentationProbe"/>.</summary>
-    private readonly PlayerPresentationProbe? _playerPresentProbe =
-        PlayerPresentationProbe.CreateFromEnvironment();
-
     public RuntimeWorldFrameEnvironmentPreparation(
         RuntimeOptions options,
         WorldTimeService worldTime,
@@ -489,10 +485,6 @@ internal sealed class RuntimeWorldFrameEnvironmentPreparation
             activeDayGroup,
             camera.Position);
 
-        // TEMPORARY #429 apparatus — one CSV row per frame; off unless
-        // ACDREAM_PROBE_PLAYER_PRESENT names a file. Strip with the fix.
-        _playerPresentProbe?.Observe(camera.Position, roots.PlayerViewPosition);
-
         UpdateSunFromSky(foundation.Sky, roots.PlayerInsideCell);
         _lighting.UpdateViewerLight(roots.PlayerViewPosition);
         _lighting.Tick(camera.Position);