fix(headless): complete connected movement gate

tests/AcDream.Core.Tests/Physics/PhysicsEngineTests.cs

@@ -31,6 +31,33 @@ public class PhysicsEngineTests
         return engine;
     }
 
+    [Fact]
+    public void Resolve_ZeroDeltaSnapTrace_IsExplicitlyOptIn()
+    {
+        var engine = new PhysicsEngine();
+        var diagnostics = new List<string>();
+
+        Assert.Null(engine.DiagnosticLog);
+        _ = engine.Resolve(
+            Vector3.Zero,
+            cellId: 0xA9B40001u,
+            delta: Vector3.Zero,
+            stepUpHeight: 0.5f);
+
+        engine.DiagnosticLog = diagnostics.Add;
+        _ = engine.Resolve(
+            Vector3.Zero,
+            cellId: 0xA9B40001u,
+            delta: Vector3.Zero,
+            stepUpHeight: 0.5f);
+
+        Assert.Single(diagnostics);
+        Assert.StartsWith(
+            "[snap] claim=0xA9B40001",
+            diagnostics[0],
+            StringComparison.Ordinal);
+    }
+
     [Fact]
     public void Resolve_FlatTerrain_ZMatchesTerrain()
     {