feat(core): Phase B.3 — CellSurface (indoor floor polygon projection)

Projects an XY point onto a cell's floor polygons via brute-force
triangle iteration + barycentric Z interpolation. Fan-triangulates
quads and larger polygons. Returns null when outside all floor
surfaces. Accepts pre-transformed world-space vertex positions so
the caller handles EnvCell coordinate transforms.

Second component of the physics collision engine.

4 new tests, all green.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

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+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class CellSurfaceTests
+{
+    /// <summary>
+    /// Build a minimal CellSurface representing a flat square floor
+    /// centered at (originX, originY) with the given half-size and Z.
+    /// The floor polygon is a quad: 4 vertices at the corners.
+    /// </summary>
+    private static CellSurface MakeFlatFloor(
+        uint cellId, float originX, float originY, float z,
+        float halfSize = 10f)
+    {
+        // 4 vertices forming a square floor at the given Z, in WORLD space.
+        var vertices = new Dictionary<ushort, Vector3>
+        {
+            [0] = new(originX - halfSize, originY - halfSize, z),
+            [1] = new(originX + halfSize, originY - halfSize, z),
+            [2] = new(originX + halfSize, originY + halfSize, z),
+            [3] = new(originX - halfSize, originY + halfSize, z),
+        };
+
+        // One quad polygon with 4 vertex IDs.
+        var polygonVertexIds = new List<List<short>>
+        {
+            new() { 0, 1, 2, 3 },
+        };
+
+        return new CellSurface(cellId, vertices, polygonVertexIds);
+    }
+
+    [Fact]
+    public void SampleFloorZ_InsideFlat_ReturnsZ()
+    {
+        var surface = MakeFlatFloor(0x0100, originX: 50f, originY: 50f, z: 10f);
+
+        float? z = surface.SampleFloorZ(50f, 50f);
+
+        Assert.NotNull(z);
+        Assert.Equal(10f, z!.Value, precision: 2);
+    }
+
+    [Fact]
+    public void SampleFloorZ_OutsideFloor_ReturnsNull()
+    {
+        var surface = MakeFlatFloor(0x0100, originX: 50f, originY: 50f, z: 10f, halfSize: 5f);
+
+        float? z = surface.SampleFloorZ(100f, 100f);
+
+        Assert.Null(z);
+    }
+
+    [Fact]
+    public void SampleFloorZ_AtEdge_ReturnsZ()
+    {
+        var surface = MakeFlatFloor(0x0100, originX: 50f, originY: 50f, z: 10f, halfSize: 10f);
+
+        // Right at the edge of the polygon.
+        float? z = surface.SampleFloorZ(60f, 50f);
+
+        Assert.NotNull(z);
+        Assert.Equal(10f, z!.Value, precision: 2);
+    }
+
+    [Fact]
+    public void SampleFloorZ_SlopedFloor_InterpolatesZ()
+    {
+        // A triangular floor that slopes from Z=0 to Z=20.
+        var vertices = new Dictionary<ushort, Vector3>
+        {
+            [0] = new(0f, 0f, 0f),
+            [1] = new(20f, 0f, 0f),
+            [2] = new(10f, 20f, 20f),
+        };
+        var polygons = new List<List<short>> { new() { 0, 1, 2 } };
+        var surface = new CellSurface(0x0100, vertices, polygons);
+
+        // At the centroid (10, 6.67): Z should be roughly 6.67
+        float? z = surface.SampleFloorZ(10f, 6.67f);
+
+        Assert.NotNull(z);
+        Assert.InRange(z!.Value, 5f, 8f);  // approximate
+    }
+}