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/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()
     {