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>

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

@@ -231,6 +231,36 @@ public class PhysicsEngineTests
         Assert.Equal(0x0025u, result.CellId);
     }
 
+    [Fact]
+    public void ResolveWithTransition_EdgeSlideStopsAtLoadedTerrainBoundary()
+    {
+        var engine = MakeFlatEngine(terrainZ: 50f);
+        var body = new PhysicsBody
+        {
+            Position = new Vector3(191.25f, 96f, 50f),
+            TransientState = TransientStateFlags.Contact | TransientStateFlags.OnWalkable,
+            ContactPlaneValid = true,
+            ContactPlane = new Plane(Vector3.UnitZ, -50f),
+            ContactPlaneCellId = 0x003Du,
+        };
+
+        var result = engine.ResolveWithTransition(
+            currentPos: new Vector3(191.25f, 96f, 50f),
+            targetPos:  new Vector3(193f, 96f, 50f),
+            cellId: 0x003Du,
+            sphereRadius: 0.5f,
+            sphereHeight: 1.2f,
+            stepUpHeight: 0.4f,
+            stepDownHeight: 0.4f,
+            isOnGround: true,
+            body: body,
+            moverFlags: ObjectInfoState.EdgeSlide);
+
+        Assert.True(result.IsOnGround);
+        Assert.InRange(result.Position.X, 190.75f, 192.0001f);
+        Assert.Equal(50f, result.Position.Z, precision: 2);
+    }
+
     [Fact]
     public void ResolveWithTransition_LandblockBoundary_UpdatesFullOutdoorCellId()
     {

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

@@ -67,6 +67,20 @@ public class TerrainSurfaceTests
         Assert.Equal(42f, surface.SampleZ(300f, 300f));
     }
 
+    [Fact]
+    public void SampleSurfacePolygon_ReturnsContainingTriangleVertices()
+    {
+        var heights = FlatHeightmap(50);
+        var surface = new TerrainSurface(heights, LinearHeightTable(), landblockX: 0, landblockY: 0);
+
+        var sample = surface.SampleSurfacePolygon(2f, 2f);
+
+        Assert.Equal(3, sample.Vertices.Length);
+        Assert.All(sample.Vertices, v => Assert.Equal(50f, v.Z));
+        Assert.Equal(1f, sample.Normal.Z, precision: 3);
+        Assert.Contains(sample.Vertices, v => v.X == 0f && v.Y == 0f);
+    }
+
     [Fact]
     public void ComputeOutdoorCellId_Origin_ReturnsFirst()
     {