feat(physics): Phase 2 — port CellTransit + wire into ResolveCellId

New CellTransit static class ports retail's portal-graph cell traversal:
- FindTransitCellsSphere — indoor portal-neighbour walk
- AddAllOutsideCells     — outdoor 24m grid expansion
- FindCellList           — top-level driver (BFS through portals;
                           PointInsideCellBsp for final containment)

PhysicsEngine.ResolveOutdoorCellId renamed to ResolveCellId. Body
rewritten: indoor seeds delegate to CellTransit.FindCellList (portal-
graph BFS + BSP containment test); outdoor seeds keep the landblock
terrain grid lookup from the original implementation (preserving the
L.2e prefix-preservation fix). Signature extended with sphereRadius
parameter (needed by the sphere-vs-portal-plane test). Three call
sites updated (PhysicsEngine x2, TransitionTypes x1).

BSPQuery.PointInsideCellBsp retyped from PhysicsBSPNode? to CellBSPNode?
— the function operates on the cell-BSP tree (CellPhysics.CellBSP.Root
is a CellBSPNode). The previous PhysicsBSPNode typing was dead code, so
retype is safe.

Deletes the Phase D ResolveOutdoorCellIdTests.cs file. New ResolveCellIdTests
covers the equivalent contracts (fallback zero, outdoor seed with no
landblock).

Outdoor->indoor entry (check_building_transit) is stubbed pending the
BuildingPhysics infrastructure landing in the next commit.

Spec: docs/superpowers/specs/2026-05-19-indoor-portal-cell-tracking-design.md
Plan: docs/superpowers/plans/2026-05-19-indoor-portal-cell-tracking.md

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

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

@@ -0,0 +1,108 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class CellTransitFindTransitCellsSphereTests
+{
+    private static CellPhysics MakeCellWithPortalAtRightWall(
+        Matrix4x4 worldTransform, uint otherCellId, ushort flags)
+    {
+        // Portal poly at local x=2.5 (right wall), normal +X.
+        var portalPolyA = new ResolvedPolygon
+        {
+            Vertices  = new[]
+            {
+                new Vector3(2.5f, -2.5f,  0f),
+                new Vector3(2.5f,  2.5f,  0f),
+                new Vector3(2.5f,  2.5f,  5f),
+                new Vector3(2.5f, -2.5f,  5f),
+            },
+            Plane     = new Plane(new Vector3(1, 0, 0), -2.5f),  // x = 2.5
+            NumPoints = 4,
+            SidesType = DatReaderWriter.Enums.CullMode.None,
+        };
+
+        Matrix4x4.Invert(worldTransform, out var inv);
+        return new CellPhysics
+        {
+            WorldTransform = worldTransform,
+            InverseWorldTransform = inv,
+            Resolved = new Dictionary<ushort, ResolvedPolygon>(),
+            PortalPolygons = new Dictionary<ushort, ResolvedPolygon> { [10] = portalPolyA },
+            Portals = new[]
+            {
+                new PortalInfo(otherCellId: (ushort)otherCellId, polygonId: 10, flags: flags),
+            },
+        };
+    }
+
+    [Fact]
+    public void SphereInsideCellA_NearPortal_AddsCellB()
+    {
+        var cellA = MakeCellWithPortalAtRightWall(Matrix4x4.Identity, otherCellId: 0x0101, flags: 0);
+
+        var cellBT = Matrix4x4.CreateTranslation(new Vector3(5f, 0f, 0f));
+        Matrix4x4.Invert(cellBT, out var cellBInv);
+        var cellB = new CellPhysics
+        {
+            WorldTransform = cellBT,
+            InverseWorldTransform = cellBInv,
+            Resolved = new Dictionary<ushort, ResolvedPolygon>(),
+        };
+
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40100u, cellA);
+        cache.RegisterCellStructForTest(0xA9B40101u, cellB);
+
+        // Sphere center near portal (local x=2.0, radius=0.5 → reaches x=2.5 = portal plane).
+        var worldSphereCenter = new Vector3(2.0f, 0f, 2.5f);
+
+        var candidates = new HashSet<uint>();
+        CellTransit.FindTransitCellsSphere(
+            cache, cellA, currentCellId: 0xA9B40100u,
+            worldSphereCenter, sphereRadius: 0.5f, candidates, out bool exitOutside);
+
+        Assert.Contains(0xA9B40101u, candidates);
+        Assert.False(exitOutside);
+    }
+
+    [Fact]
+    public void SphereInsideCellA_FarFromPortal_DoesNotAddCellB()
+    {
+        var cellA = MakeCellWithPortalAtRightWall(Matrix4x4.Identity, otherCellId: 0x0101, flags: 0);
+
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40100u, cellA);
+
+        // Sphere far from portal (local x=-1.0, reach to x=-0.5 — nowhere near portal at x=2.5).
+        var worldSphereCenter = new Vector3(-1.0f, 0f, 2.5f);
+
+        var candidates = new HashSet<uint>();
+        CellTransit.FindTransitCellsSphere(
+            cache, cellA, currentCellId: 0xA9B40100u,
+            worldSphereCenter, sphereRadius: 0.5f, candidates, out bool exitOutside);
+
+        Assert.DoesNotContain(0xA9B40101u, candidates);
+    }
+
+    [Fact]
+    public void ExitPortal_SphereStraddlesPortalPlane_FlagsCheckOutside()
+    {
+        var exitCell = MakeCellWithPortalAtRightWall(Matrix4x4.Identity, otherCellId: 0xFFFF, flags: 0);
+
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40100u, exitCell);
+
+        var worldSphereCenter = new Vector3(2.0f, 0f, 2.5f);
+        var candidates = new HashSet<uint>();
+
+        CellTransit.FindTransitCellsSphere(
+            cache, exitCell, currentCellId: 0xA9B40100u,
+            worldSphereCenter, sphereRadius: 0.5f, candidates, out bool exitOutside);
+
+        Assert.True(exitOutside);
+    }
+}