feat(core): Phase B.3 — PortalPlane (plane math + crossing detection)

Adds the foundational portal-plane record for cell transition detection.
PortalPlane.FromVertices computes a normalised plane from 3 coplanar
polygon vertices via cross product + dot product; IsCrossing tests whether
a movement vector straddles the plane (strictly negative dot-product
product — exact-on-plane position returns false as specified).

4 new unit tests: normal construction, opposite-side crossing, same-side
no-crossing, start-on-plane no-crossing.  All 269 tests green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,67 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class PortalPlaneTests
+{
+    // Helper: build an XY-plane portal (z = 0) from three known vertices.
+    private static PortalPlane XyPlane() =>
+        PortalPlane.FromVertices(
+            new Vector3(0, 0, 0),
+            new Vector3(1, 0, 0),
+            new Vector3(0, 1, 0),
+            targetCellId: 42,
+            ownerCellId: 7,
+            flags: 0);
+
+    [Fact]
+    public void FromVertices_ComputesCorrectNormal()
+    {
+        var plane = XyPlane();
+
+        // Cross((1,0,0)-(0,0,0), (0,1,0)-(0,0,0)) = Cross((1,0,0),(0,1,0)) = (0,0,1)
+        // The normalised result must be either (0,0,1) or (0,0,-1).
+        Assert.Equal(0f, plane.Normal.X, precision: 5);
+        Assert.Equal(0f, plane.Normal.Y, precision: 5);
+        Assert.Equal(1f, MathF.Abs(plane.Normal.Z), precision: 5);
+    }
+
+    [Fact]
+    public void IsCrossing_PositionsOnOppositeSides_ReturnsTrue()
+    {
+        var plane = XyPlane(); // normal is (0,0,±1), D = 0
+
+        // One position above the XY plane, one below.
+        var above = new Vector3(0, 0, 1f);
+        var below = new Vector3(0, 0, -1f);
+
+        Assert.True(plane.IsCrossing(above, below));
+        Assert.True(plane.IsCrossing(below, above));
+    }
+
+    [Fact]
+    public void IsCrossing_PositionsOnSameSide_ReturnsFalse()
+    {
+        var plane = XyPlane();
+
+        var pos1 = new Vector3(0, 0, 1f);
+        var pos2 = new Vector3(0, 0, 2f);
+
+        Assert.False(plane.IsCrossing(pos1, pos2));
+    }
+
+    [Fact]
+    public void IsCrossing_StartOnPlane_ReturnsFalse()
+    {
+        var plane = XyPlane();
+
+        // oldPos is exactly on the plane (z = 0) → distance = 0.
+        // Product (0 * anything) == 0, which is NOT < 0 → no crossing.
+        var onPlane = new Vector3(0.5f, 0.5f, 0f);
+        var above   = new Vector3(0.5f, 0.5f, 1f);
+
+        Assert.False(plane.IsCrossing(onPlane, above));
+    }
+}