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>

src/AcDream.Core/Physics/PortalPlane.cs

@@ -0,0 +1,45 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// A portal plane derived from an EnvCell's CellPortal polygon.
+/// Used to detect when a player crosses from one cell into another.
+/// </summary>
+public readonly record struct PortalPlane(
+    Vector3 Normal,
+    float D,
+    uint TargetCellId,   // OtherCellId — the cell on the far side (0xFFFF = outdoor)
+    uint OwnerCellId,    // the EnvCell that owns this portal
+    ushort Flags)        // PortalFlags value
+{
+    /// <summary>
+    /// Construct a PortalPlane from three coplanar vertices (winding order
+    /// determines the normal direction via cross product).
+    /// </summary>
+    public static PortalPlane FromVertices(
+        Vector3 v0, Vector3 v1, Vector3 v2,
+        uint targetCellId, uint ownerCellId, ushort flags)
+    {
+        var edge1 = v1 - v0;
+        var edge2 = v2 - v0;
+        var normal = Vector3.Normalize(Vector3.Cross(edge1, edge2));
+        float d = -Vector3.Dot(normal, v0);
+        return new PortalPlane(normal, d, targetCellId, ownerCellId, flags);
+    }
+
+    /// <summary>
+    /// Returns true when the movement from <paramref name="oldPos"/> to
+    /// <paramref name="newPos"/> crosses this plane (the two positions are on
+    /// strictly opposite sides).  A position exactly on the plane (distance = 0)
+    /// does NOT count as a crossing.
+    /// </summary>
+    public bool IsCrossing(Vector3 oldPos, Vector3 newPos)
+    {
+        float oldDist = Vector3.Dot(Normal, oldPos) + D;
+        float newDist = Vector3.Dot(Normal, newPos) + D;
+        // Strictly negative product → opposite signs → crossed the plane.
+        // If either distance is exactly 0 the product is 0, not negative → no crossing.
+        return oldDist * newDist < 0f;
+    }
+}