fix(physics): #30 #34 L.2a movement truth diagnostics

Pass explicit grounded/airborne contact bytes from MovementResult into MoveToState and AutonomousPosition, and add ACDREAM_DUMP_MOVE_TRUTH logging for outbound movement plus player UpdatePosition echoes.

Co-authored-by: OpenAI Codex <codex@openai.com>

tests/AcDream.Core.Net.Tests/Messages/AutonomousPositionTests.cs

@@ -105,6 +105,23 @@ public class AutonomousPositionTests
         Assert.Equal(56, body.Length);
     }
 
+    [Fact]
+    public void Build_UsesExplicitAirborneContactByte()
+    {
+        var body = AutonomousPosition.Build(
+            gameActionSequence: 7,
+            cellId: 0xA9B40001u,
+            position: Vector3.Zero,
+            rotation: Quaternion.Identity,
+            instanceSequence: 0,
+            serverControlSequence: 0,
+            teleportSequence: 0,
+            forcePositionSequence: 0,
+            lastContact: 0);
+
+        Assert.Equal(0, body[52]);
+    }
+
     [Fact]
     public void Build_ContainsIdentityRotation_AfterPosition()
     {