test(physics): MyRunRate fallback test for GetMaxSpeed (L.3.1 Task 2 polish)

Code-quality review on commit 9c5634a flagged that the existing 4
GetMaxSpeed tests didn't cover the case where WeenieObj is null and
RunForward must fall back to MyRunRate. Without this test, a
regression that hardcoded the fallback to 1.0f would silently pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -873,4 +873,19 @@ public sealed class MotionInterpreterTests
 
         Assert.Equal(0f, speed, precision: 4);
     }
+
+    [Fact]
+    public void GetMaxSpeed_RunForward_NoWeenie_FallsBackToMyRunRate()
+    {
+        // WeenieObj is null (MakeInterp with no weenie argument); MyRunRate
+        // is set explicitly. GetMaxSpeed must use MyRunRate as the run-rate
+        // source when InqRunRate is unavailable.
+        var interp = MakeInterp();
+        interp.MyRunRate = 1.75f;
+        interp.InterpretedState.ForwardCommand = MotionCommand.RunForward;
+
+        float speed = interp.GetMaxSpeed();
+
+        Assert.Equal(MotionInterpreter.RunAnimSpeed * 1.75f, speed, precision: 4);
+    }
 }