feat(B.6 slice 1): ACDREAM_PROBE_AUTOWALK diagnostic baseline

Per the B.6 design spec (now retail-grounded on Option A), slice 1 is
pure-additive logging so the next session has a clean trace of what
ACE actually sends to the local player during a server-initiated
auto-walk.

New PhysicsDiagnostics.ProbeAutoWalkEnabled static flag, env-var-
initialized from ACDREAM_PROBE_AUTOWALK=1. Probe sites:

  [autowalk-out] on SendUse + SendPickUp — the packets that trigger
      ACE's CreateMoveToChain when the target is out of WithinUseRadius.
  [autowalk-mt]  on OnLiveMotionUpdated for _playerServerGuid only —
      captures MovementType + MoveToPath origin/min-dist/obj-dist +
      moveTowards + speed/runRate. Lets us see exactly the wire data
      retail's PerformMovement case 6 (0x00524440) was acting on.
  [autowalk-up]  on OnLivePositionUpdated for _playerServerGuid only —
      cadence + payload of ACE's position broadcasts during auto-walk.

No behavior change. All flags off by default; opt in with the env var
during a focused reproduction. Designed to be mirrored into DebugVM
checkbox state later (parallel to ProbeResolve / ProbeCell / ProbeBuilding)
but not wired yet — env-var-only for the first trace session.

src/AcDream.App/Rendering/GameWindow.cs

@@ -3290,6 +3290,29 @@ public sealed class GameWindow : IDisposable
             {
                 // Still update the stance echo (_playerMotionTableId, etc) via
                 // the paths above, but don't stomp the animation sequencer.
+
+                // B.6 slice 1 (2026-05-14): trace inbound motion for the
+                // local player so we can characterize what ACE sends during
+                // a server-initiated auto-walk. One line per inbound UM,
+                // gated on ACDREAM_PROBE_AUTOWALK=1.
+                if (AcDream.Core.Physics.PhysicsDiagnostics.ProbeAutoWalkEnabled)
+                {
+                    string cmdHex = command.HasValue ? $"0x{command.Value:X4}" : "null";
+                    string pathStr = update.MotionState.MoveToPath is { } p
+                        ? $"path=cell=0x{p.OriginCellId:X8},xyz=({p.OriginX:F2},{p.OriginY:F2},{p.OriginZ:F2}),minDist={p.MinDistance:F2},objDist={p.DistanceToObject:F2}"
+                        : "path=null";
+                    string spd = update.MotionState.ForwardSpeed is { } fs
+                        ? $"fwdSpd={fs:F2}"
+                        : "fwdSpd=null";
+                    string mtsSpd = update.MotionState.MoveToSpeed is { } ms
+                        ? $"mtSpd={ms:F2}"
+                        : "mtSpd=null";
+                    string mtsRun = update.MotionState.MoveToRunRate is { } mr
+                        ? $"mtRun={mr:F2}"
+                        : "mtRun=null";
+                    Console.WriteLine(System.FormattableString.Invariant(
+                        $"[autowalk-mt] stance=0x{stance:X4} cmd={cmdHex} mt=0x{update.MotionState.MovementType:X2} isMoveTo={update.MotionState.IsServerControlledMoveTo} moveTowards={update.MotionState.MoveTowards} {pathStr} {spd} {mtsSpd} {mtsRun}"));
+                }
             }
             else
             {
@@ -4149,6 +4172,22 @@ public sealed class GameWindow : IDisposable
             (lbY - _liveCenterY) * 192f,
             0f);
         var worldPos = new System.Numerics.Vector3(p.PositionX, p.PositionY, p.PositionZ) + origin;
+
+        // B.6 slice 1 (2026-05-14): trace inbound UpdatePosition cadence for
+        // the local player. Combined with [autowalk-mt] this answers
+        // whether ACE's broadcast frequency during a server-initiated
+        // auto-walk is dense enough to drive smooth visible motion (the
+        // Option C viability check from the design spec). Gated on
+        // ACDREAM_PROBE_AUTOWALK=1; skips remote entities.
+        if (update.Guid == _playerServerGuid
+            && AcDream.Core.Physics.PhysicsDiagnostics.ProbeAutoWalkEnabled)
+        {
+            string velStr = update.Velocity is { } v
+                ? $"vel=({v.X:F2},{v.Y:F2},{v.Z:F2})"
+                : "vel=null";
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[autowalk-up] cell=0x{p.LandblockId:X8} pos=({p.PositionX:F2},{p.PositionY:F2},{p.PositionZ:F2}) world=({worldPos.X:F2},{worldPos.Y:F2},{worldPos.Z:F2}) {velStr} grounded={update.IsGrounded}"));
+        }
         var rot = new System.Numerics.Quaternion(p.RotationX, p.RotationY, p.RotationZ, p.RotationW);
         DumpMovementTruthServerEcho(update, worldPos);
 
@@ -8896,6 +8935,12 @@ public sealed class GameWindow : IDisposable
         var body = AcDream.Core.Net.Messages.InteractRequests.BuildUse(seq, guid);
         _liveSession.SendGameAction(body);
         Console.WriteLine($"[B.4b] use guid=0x{guid:X8} seq={seq}");
+        if (AcDream.Core.Physics.PhysicsDiagnostics.ProbeAutoWalkEnabled)
+        {
+            string label = DescribeLiveEntity(guid);
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[autowalk-out] op=use target=0x{guid:X8} name=\"{label}\" seq={seq}"));
+        }
     }
 
     private void SendPickUp(uint itemGuid)
@@ -8925,6 +8970,12 @@ public sealed class GameWindow : IDisposable
             seq, itemGuid, _playerServerGuid, placement: 0);
         _liveSession.SendGameAction(body);
         Console.WriteLine($"[B.5] pickup item=0x{itemGuid:X8} container=0x{_playerServerGuid:X8} seq={seq}");
+        if (AcDream.Core.Physics.PhysicsDiagnostics.ProbeAutoWalkEnabled)
+        {
+            string label = DescribeLiveEntity(itemGuid);
+            Console.WriteLine(System.FormattableString.Invariant(
+                $"[autowalk-out] op=pickup target=0x{itemGuid:X8} name=\"{label}\" seq={seq}"));
+        }
     }
 
     private uint? SelectClosestCombatTarget(bool showToast)

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -94,4 +94,30 @@ public static class PhysicsDiagnostics
     /// </para>
     /// </summary>
     public static ResolvedPolygon? LastBspHitPoly { get; set; }
+
+    /// <summary>
+    /// B.6 slice 1 (2026-05-14) — baseline trace for the local-player
+    /// server-initiated auto-walk path (issue #63). When true, the
+    /// following events emit one-line <c>[autowalk-*]</c> logs:
+    /// <list type="bullet">
+    ///   <item><description><c>[autowalk-out]</c> on every <c>SendUse</c>
+    ///     / <c>SendPickUp</c> the local player issues — these are the
+    ///     packets that may trigger ACE's server-side <c>CreateMoveToChain</c>
+    ///     when the target is out of <c>WithinUseRadius</c>.</description></item>
+    ///   <item><description><c>[autowalk-mt]</c> on every inbound
+    ///     <c>UpdateMotion</c> for the local player — captures the
+    ///     <c>MovementType + MoveToPath + speed/runRate</c> ACE sends.</description></item>
+    ///   <item><description><c>[autowalk-up]</c> on every inbound
+    ///     <c>UpdatePosition</c> for the local player — answers "what's
+    ///     ACE's broadcast cadence during auto-walk?"</description></item>
+    /// </list>
+    /// Initial state from <c>ACDREAM_PROBE_AUTOWALK=1</c>.
+    ///
+    /// <para>
+    /// Spec: <c>docs/superpowers/specs/2026-05-14-phase-b6-design.md</c>
+    /// §"Required investigation".
+    /// </para>
+    /// </summary>
+    public static bool ProbeAutoWalkEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_AUTOWALK") == "1";
 }