fix(physics): hold retail cell across inner retries

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -385,33 +385,97 @@ public sealed class PhysicsEngine
             float localX = worldX - lb.WorldOffsetX;
             float localY = worldY - lb.WorldOffsetY;
             if (localX >= 0f && localX < 192f && localY >= 0f && localY < 192f)
-            {
-                var sample = lb.Terrain.SampleSurfacePolygon(localX, localY);
-                var vertices = new TerrainTriangleVertices(
-                    OffsetTerrainVertex(sample.Vertices.V0, lb),
-                    OffsetTerrainVertex(sample.Vertices.V1, lb),
-                    OffsetTerrainVertex(sample.Vertices.V2, lb));
-
-                var normal = sample.Normal;
-                float d = -Vector3.Dot(normal, vertices[0]);
-                var plane = new System.Numerics.Plane(normal, d);
-
-                float waterDepth = lb.Terrain.SampleWaterDepth(localX, localY);
-                bool isWater = waterDepth >= 0.45f;
-                uint lowCellId = lb.Terrain.ComputeOutdoorCellId(localX, localY);
-                uint fullCellId = (kvp.Key & 0xFFFF0000u) | lowCellId;
-
-                return new TerrainWalkableSample(
-                    plane,
-                    vertices,
-                    waterDepth,
-                    isWater,
-                    fullCellId);
-            }
+                return BuildTerrainWalkableSample(kvp.Key, lb, localX, localY);
         }
         return null;
     }
 
+    /// <summary>
+    /// Samples only the fixed outdoor cell supplied to retail
+    /// <c>CTransition::insert_into_cell</c>. The target point may move into a
+    /// neighboring cell during a retry, but retail continues dispatching the
+    /// captured <c>CObjCell*</c> until that inner call returns.
+    /// </summary>
+    internal TerrainWalkableSample? SampleTerrainWalkableInCell(
+        uint cellId,
+        float worldX,
+        float worldY)
+    {
+        uint lowCellId = cellId & 0xFFFFu;
+        if (lowCellId is < 1u or > 0x40u)
+            return null;
+
+        foreach (var kvp in _landblocks)
+        {
+            uint requestedPrefix = cellId & 0xFFFF0000u;
+            if (requestedPrefix != 0u &&
+                (kvp.Key & 0xFFFF0000u) != requestedPrefix)
+                continue;
+
+            LandblockPhysics lb = kvp.Value;
+            float localX = worldX - lb.WorldOffsetX;
+            float localY = worldY - lb.WorldOffsetY;
+            if (requestedPrefix == 0u &&
+                (localX < 0f || localX >= 192f ||
+                 localY < 0f || localY >= 192f))
+            {
+                continue;
+            }
+            int cellIndex = (int)lowCellId - 1;
+            int cellX = cellIndex / TerrainSurface.CellsPerSide;
+            int cellY = cellIndex % TerrainSurface.CellsPerSide;
+            float minX = cellX * TerrainSurface.CellSize;
+            float minY = cellY * TerrainSurface.CellSize;
+            float maxX = minX + TerrainSurface.CellSize;
+            float maxY = minY + TerrainSurface.CellSize;
+
+            if (localX < minX || localX >= maxX ||
+                localY < minY || localY >= maxY)
+            {
+                return null;
+            }
+
+            return BuildTerrainWalkableSample(
+                kvp.Key,
+                lb,
+                localX,
+                localY);
+        }
+
+        return null;
+    }
+
+    private static TerrainWalkableSample BuildTerrainWalkableSample(
+        uint landblockId,
+        LandblockPhysics landblock,
+        float localX,
+        float localY)
+    {
+        TerrainSurfacePolygon sample = landblock.Terrain.SampleSurfacePolygon(
+            localX,
+            localY);
+        var vertices = new TerrainTriangleVertices(
+            OffsetTerrainVertex(sample.Vertices.V0, landblock),
+            OffsetTerrainVertex(sample.Vertices.V1, landblock),
+            OffsetTerrainVertex(sample.Vertices.V2, landblock));
+
+        Vector3 normal = sample.Normal;
+        float d = -Vector3.Dot(normal, vertices[0]);
+        var plane = new System.Numerics.Plane(normal, d);
+
+        float waterDepth = landblock.Terrain.SampleWaterDepth(localX, localY);
+        bool isWater = waterDepth >= 0.45f;
+        uint lowCellId = landblock.Terrain.ComputeOutdoorCellId(localX, localY);
+        uint fullCellId = (landblockId & 0xFFFF0000u) | lowCellId;
+
+        return new TerrainWalkableSample(
+            plane,
+            vertices,
+            waterDepth,
+            isWater,
+            fullCellId);
+    }
+
     private static Vector3 OffsetTerrainVertex(Vector3 vertex, LandblockPhysics landblock)
         => new(
             vertex.X + landblock.WorldOffsetX,
@@ -466,11 +530,11 @@ public sealed class PhysicsEngine
     }
 
     /// <summary>
-    /// TEST-ONLY outdoor cell re-derive. The single caller is
-    /// <c>Transition.FindEnvCollisions</c>'s cache-null fallback
+    /// TEST-ONLY outdoor cell re-derive. The sole caller is
+    /// <c>Transition.RunCheckOtherCellsAndAdvance</c>'s cache-null fallback
     /// (PhysicsEngineTests run engines without a <see cref="DataCache"/>,
-    /// so <see cref="CellTransit.FindCellSet"/> is unavailable). Production
-    /// membership flows exclusively through the collide-then-pick advance
+    /// so <see cref="CellTransit.FindCellSet"/> is unavailable). Normal
+    /// production membership flows exclusively through the collide-then-pick advance
     /// (<c>RunCheckOtherCellsAndAdvance</c> → <c>FindCellSet</c>).
     ///
     /// <para>