feat(physics): A4 — Transition.CheckOtherCells + ApplyOtherCellResult

Port of retail's CTransition::check_other_cells at
acclient_2013_pseudo_c.txt:272717-272798. Iterates every non-primary
cell in a candidate set, runs BSPQuery.FindCollisions per cell with
that cell's WorldTransform-derived rotation + origin, halts on first
Collided/Adjusted/Slid.

ApplyOtherCellResult is the combine-semantics helper extracted for
unit testability — it pins the retail switch:
  - Collided/Adjusted → CollidedWithEnvironment = true (gated on
    !Contact), halt.
  - Slid              → ContactPlaneValid + ContactPlaneIsWater = false,
                        halt.
  - OK                → continue.

Not yet wired into FindEnvCollisions — see next commit. Probe gated
on PhysicsDiagnostics.ProbeIndoorBspEnabled (ACDREAM_PROBE_INDOOR_BSP).

Six new unit tests: five against the pure combine helper for each halt
case + one direct CheckOtherCells call exercising the null-BSP guard.

Spec: docs/superpowers/specs/2026-05-20-phase-a4-multi-cell-bsp-design.md
Plan: docs/superpowers/plans/2026-05-20-phase-a4-multi-cell-bsp.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,142 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+/// <summary>
+/// Unit tests for the result-combine helper used by
+/// <see cref="Transition.CheckOtherCells"/>. The iteration / per-cell
+/// BSP-query parts are covered end-to-end by
+/// <see cref="FindEnvCollisionsMultiCellTests"/>; this file pins the
+/// retail-faithful halt semantics that
+/// <c>acclient_2013_pseudo_c.txt:272739-272752</c> spells out.
+/// </summary>
+public class TransitionCheckOtherCellsTests
+{
+    private static Transition MakeTransition(bool contactFlag = false)
+    {
+        var t = new Transition();
+        t.SpherePath.InitPath(Vector3.Zero, Vector3.Zero, cellId: 0xA9B40100u, sphereRadius: 0.48f);
+        t.ObjectInfo.State = contactFlag ? ObjectInfoState.Contact : ObjectInfoState.None;
+        // Pre-set CP fields to non-default so the Slid-clears-CP assertion
+        // can detect the clear.
+        t.CollisionInfo.ContactPlaneValid   = true;
+        t.CollisionInfo.ContactPlaneIsWater = true;
+        return t;
+    }
+
+    [Fact]
+    public void OK_ContinuesIteration_DoesNotMutate()
+    {
+        var t = MakeTransition();
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.OK, out var finalState);
+
+        Assert.False(halt);
+        Assert.Equal(TransitionState.OK, finalState);
+        Assert.True(t.CollisionInfo.ContactPlaneValid);
+        Assert.True(t.CollisionInfo.ContactPlaneIsWater);
+        Assert.False(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Collided_HaltsAndSetsCollidedWithEnvironment_WhenNotInContact()
+    {
+        var t = MakeTransition(contactFlag: false);
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Collided, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Collided, finalState);
+        Assert.True(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Collided_DoesNotSetCollidedWithEnvironment_WhenInContact()
+    {
+        // Retail oracle gating: the CollidedWithEnvironment flip mirrors
+        // the existing primary-cell behavior in FindEnvCollisions —
+        // skipped when ObjectInfo.State has Contact bit set.
+        var t = MakeTransition(contactFlag: true);
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Collided, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Collided, finalState);
+        Assert.False(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Adjusted_HaltsAndSetsCollidedWithEnvironment_WhenNotInContact()
+    {
+        var t = MakeTransition(contactFlag: false);
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Adjusted, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Adjusted, finalState);
+        Assert.True(t.CollisionInfo.CollidedWithEnvironment);
+    }
+
+    [Fact]
+    public void Slid_HaltsAndClearsContactPlaneFields()
+    {
+        // Retail oracle: acclient_2013_pseudo_c.txt:272746-272750
+        //   case 4:
+        //     this->collision_info.contact_plane_valid = 0;
+        //     this->collision_info.contact_plane_is_water = 0;
+        //     return result;
+        var t = MakeTransition();
+        Assert.True(t.CollisionInfo.ContactPlaneValid);    // pre-condition
+        Assert.True(t.CollisionInfo.ContactPlaneIsWater);  // pre-condition
+
+        bool halt = t.ApplyOtherCellResult(TransitionState.Slid, out var finalState);
+
+        Assert.True(halt);
+        Assert.Equal(TransitionState.Slid, finalState);
+        Assert.False(t.CollisionInfo.ContactPlaneValid);
+        Assert.False(t.CollisionInfo.ContactPlaneIsWater);
+    }
+
+    [Fact]
+    public void CheckOtherCells_CellWithNullBspRoot_IsSkippedNoCrash()
+    {
+        // Iteration safety: a CellPhysics in the candidate set with
+        // `BSP = null` (loaded for render but not physics) must be skipped,
+        // not crash. Matches the spec's R2 guard at design §Edge cases E2.
+        var cell = new CellPhysics
+        {
+            BSP                   = null,  // <-- the guard target
+            WorldTransform        = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
+        };
+
+        var engine = new PhysicsEngine();
+        engine.DataCache = new PhysicsDataCache();   // PhysicsEngine has nullable DataCache
+        // FindEnvCollisions has terrain probes downstream; populate a
+        // minimal landblock so the cache + engine are coherent. The cell
+        // we test against doesn't need a real landblock entry.
+        var heights = new byte[81];
+        Array.Fill(heights, (byte)0);
+        var ht = new float[256];
+        for (int i = 0; i < 256; i++) ht[i] = i * 1.0f;
+        engine.AddLandblock(0xA9B4FFFFu, new TerrainSurface(heights, ht),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(),
+            worldOffsetX: 0f, worldOffsetY: 0f);
+        engine.DataCache.RegisterCellStructForTest(0xA9B40157u, cell);
+
+        var t = MakeTransition();
+        var cellSet = new HashSet<uint> { 0xA9B40157u };
+
+        // Call CheckOtherCells directly via the internal seam.
+        var result = t.CheckOtherCells(engine, Vector3.Zero, 0.48f, cellSet);
+
+        Assert.Equal(TransitionState.OK, result);
+    }
+}