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/ResolveCellIdTests.cs

@@ -0,0 +1,31 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class ResolveCellIdTests
+{
+    [Fact]
+    public void ResolveCellId_FallbackZero_ReturnsZero()
+    {
+        var engine = new PhysicsEngine();
+        uint result = engine.ResolveCellId(Vector3.Zero, sphereRadius: 0.5f, fallbackCellId: 0u);
+        Assert.Equal(0u, result);
+    }
+
+    [Fact]
+    public void ResolveCellId_NoLandblock_OutdoorSeed_ReturnsFallback()
+    {
+        var engine = new PhysicsEngine();
+        engine.DataCache = new PhysicsDataCache();
+        // Outdoor seed with no landblock added → AddAllOutsideCells produces
+        // candidates but none have a CellBSP → falls back to input.
+        uint result = engine.ResolveCellId(
+            new Vector3(100, 100, 0),
+            sphereRadius: 0.5f,
+            fallbackCellId: 0xA9B40001u);
+
+        Assert.Equal(0xA9B40001u, result);
+    }
+}