fix(physics): #32 L.2c precipice edge-slide context

Port the first retail precipice-slide slice from named retail/ACE: terrain and BSP walkable hits now preserve polygon vertices, failed step-down edges back-probe to rediscover the walkable polygon, and edge-slide can run precipice/cliff slide instead of only hard-stopping.

Adds pseudocode anchors plus regression coverage for terrain polygon context and loaded-terrain boundary edge-slide.

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

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -4,6 +4,13 @@ using System.Numerics;
 
 namespace AcDream.Core.Physics;
 
+internal readonly record struct TerrainWalkableSample(
+    System.Numerics.Plane Plane,
+    Vector3[] Vertices,
+    float WaterDepth,
+    bool IsWater,
+    uint CellId);
+
 /// <summary>
 /// Top-level physics resolver that combines <see cref="TerrainSurface"/> and
 /// <see cref="CellSurface"/> to resolve entity movement with step-height
@@ -162,6 +169,51 @@ public sealed class PhysicsEngine
         return null;
     }
 
+    /// <summary>
+    /// Sample the outdoor terrain walkable triangle at the given world-space
+    /// XY position. This carries the same plane as <see cref="SampleTerrainPlane"/>
+    /// plus world-space triangle vertices for retail precipice-slide.
+    /// </summary>
+    internal TerrainWalkableSample? SampleTerrainWalkable(float worldX, float worldY)
+    {
+        foreach (var kvp in _landblocks)
+        {
+            var lb = kvp.Value;
+            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 Vector3[sample.Vertices.Length];
+                for (int i = 0; i < sample.Vertices.Length; i++)
+                {
+                    var v = sample.Vertices[i];
+                    vertices[i] = new Vector3(
+                        v.X + lb.WorldOffsetX,
+                        v.Y + lb.WorldOffsetY,
+                        v.Z);
+                }
+
+                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 null;
+    }
+
     /// <summary>
     /// Resolve the outdoor cell id that owns a world-space position.
     /// Indoor ids are preserved because EnvCell ownership still comes from