fix(physics): port retail stair edge backprobe

tests/AcDream.Core.Tests/Physics/Issue185OutdoorStairsSeamReplayTests.cs

@@ -177,6 +177,48 @@ public class Issue185OutdoorStairsSeamReplayTests
             "A continuous walkable ramp seam must not persist a horizontal sliding normal (#137 family).");
     }
 
+    /// <summary>
+    /// #271 live capture, quantum 310: a forward/uphill displacement that also
+    /// presses into the staircase's side wall must keep its uphill tangent.
+    /// Pre-fix the composite retry path reversed that tangent, moving from
+    /// Y=75.539 to Y=75.199 and rapidly carrying the player back down the stairs.
+    /// </summary>
+    [Fact]
+    public void OutdoorStairs_SideWallContact_DoesNotReverseUphillTangent()
+    {
+        var engine = BuildStairEngine();
+        var body = GroundedOnTread();
+        body.Position = new Vector3(133.03775f, 75.53931f, 59.608147f);
+        body.ContactPlane = new Plane(
+            new Vector3(3.2782555e-07f, -0.62469506f, 0.78086877f),
+            0.75193405f);
+
+        ResolveResult result = engine.ResolveWithTransition(
+            currentPos: body.Position,
+            targetPos: body.Position + new Vector3(0.30007935f, 0.8837738f, 0f),
+            cellId: StairCellId,
+            sphereRadius: 0.48f,
+            sphereHeight: 1.835f,
+            stepUpHeight: 0.6f,
+            stepDownHeight: 1.5f,
+            isOnGround: true,
+            body: body,
+            moverFlags: ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide,
+            movingEntityId: 0x01000000u);
+
+        _out.WriteLine(
+            $"out=({result.Position.X:F6},{result.Position.Y:F6},{result.Position.Z:F6}) " +
+            $"collision={result.CollisionNormalValid} " +
+            $"normal=({result.CollisionNormal.X:F3},{result.CollisionNormal.Y:F3},{result.CollisionNormal.Z:F3})");
+
+        Assert.True(result.Position.Y > body.Position.Y + 0.25f,
+            $"Side-wall response failed to preserve meaningful uphill motion: " +
+            $"{body.Position.Y:F6} -> {result.Position.Y:F6}.");
+        Assert.True(result.Position.Z >= body.Position.Z - 0.001f,
+            $"Side-wall response dropped the grounded player downhill: " +
+            $"{body.Position.Z:F6} -> {result.Position.Z:F6}.");
+    }
+
     private static string SolutionRoot()
     {
         var dir = AppContext.BaseDirectory;