Reapply "feat(physics): A4 — wire CheckOtherCells into FindEnvCollisions"

This reverts commit 3add110449.

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -1502,7 +1502,7 @@ public sealed class Transition
         }
     }
 
-    private TransitionState FindEnvCollisions(PhysicsEngine engine)
+    internal TransitionState FindEnvCollisions(PhysicsEngine engine)
     {
         var sp = SpherePath;
         var ci = CollisionInfo;
@@ -1618,6 +1618,21 @@ public sealed class Transition
                     return cellState;
                 }
 
+                // ── Phase A4 (2026-05-20): query every other cell ──────────
+                // Retail oracle: CTransition::check_other_cells at
+                // acclient_2013_pseudo_c.txt:272717-272798. The vestibule
+                // walls bug (cell 0xA9B40164 has only 4 polys; adjacent
+                // 0xA9B40157 has the actual walls) closes here.
+                //
+                // Discard the containing-cell return — sp.CheckCellId is
+                // already authoritative for the primary cell we just queried.
+                _ = CellTransit.FindCellSet(engine.DataCache, footCenter, sphereRadius,
+                                            sp.CheckCellId, out var cellSet);
+                var otherCellsState = CheckOtherCells(engine, footCenter, sphereRadius, cellSet);
+                if (otherCellsState != TransitionState.OK)
+                    return otherCellsState;
+                // ──────────────────────────────────────────────────────────
+
                 // ── Synthesize indoor walkable contact plane ──────────────
                 // Indoor walking Phase 2 follow-up (2026-05-19). When the BSP
                 // returns OK (no wall collision), the player is standing on a

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

@@ -0,0 +1,148 @@
+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>
+/// End-to-end test that the indoor branch of
+/// <see cref="Transition.FindEnvCollisions"/> queries the cells the
+/// sphere overlaps, not just the cell whose CellBSP contains the
+/// sphere center. This is the core Phase A4 behaviour test — the
+/// Holtburg inn vestibule (cell 0xA9B40164) bug reduced to a minimal
+/// synthetic fixture.
+/// </summary>
+public class FindEnvCollisionsMultiCellTests
+{
+    // Indoor cell IDs — both have low-byte ≥ 0x100 to trigger the
+    // indoor branch of FindEnvCollisions. Vestibule has the lower id so
+    // CellTransit.FindCellSet's sorted iteration encounters it first.
+    private const uint VestibuleCellId = 0xA9B40157u;
+    private const uint InteriorCellId  = 0xA9B40164u;
+
+    private static CellBSPTree LeafCellBsp() => new CellBSPTree
+    {
+        Root = new CellBSPNode { Type = BSPNodeType.Leaf },
+    };
+
+    private static PhysicsBSPTree EmptyLeafBsp() => new PhysicsBSPTree
+    {
+        Root = new PhysicsBSPNode
+        {
+            Type           = BSPNodeType.Leaf,
+            BoundingSphere = new Sphere { Origin = Vector3.Zero, Radius = 10f },
+        }
+    };
+
+    [Fact]
+    public void IndoorSphereOverlappingAdjacentCellWithWall_HaltsTransition()
+    {
+        // ── Secondary cell (interior) ─────────────────────────────────────
+        // Reuse BSPStepUp's TallWall fixture — proven to halt a grounded mover
+        // that can't scale it (test B2_GroundedMover_TallWall_BlockedOrSlides).
+        // Wall is at interior-local x=0.5. Translate the interior cell by
+        // +0.3 in world X so the wall ends up at world x=0.8, within reach
+        // of a sphere walking from x=0.1 toward x=0.6 (sphere radius 0.2).
+        var (wallRoot, wallResolved) = BSPStepUpFixtures.TallWall();
+        var interiorWT  = Matrix4x4.CreateTranslation(new Vector3(0.3f, 0f, 0f));
+        Matrix4x4.Invert(interiorWT, out var interiorInv);
+
+        var interior = new CellPhysics
+        {
+            BSP                   = new PhysicsBSPTree { Root = wallRoot },
+            WorldTransform        = interiorWT,
+            InverseWorldTransform = interiorInv,
+            Resolved              = wallResolved,
+            CellBSP               = LeafCellBsp(),
+        };
+
+        // ── Primary cell (vestibule) ──────────────────────────────────────
+        // Empty PhysicsBSP — no walls of its own. CellBSP contains a portal
+        // at world x=0.5 (vestibule-local x=0.5 since vestibule WorldTransform
+        // = Identity) leading to the interior cell. The sphere's foot reaches
+        // the portal at world x=0.5 during the sweep — that triggers
+        // CellTransit.FindCellSet to add the interior to the candidate set.
+        var portalPoly = new ResolvedPolygon
+        {
+            Vertices  = new[]
+            {
+                new Vector3(0.5f, -2.5f, 0f),
+                new Vector3(0.5f,  2.5f, 0f),
+                new Vector3(0.5f,  2.5f, 5f),
+                new Vector3(0.5f, -2.5f, 5f),
+            },
+            Plane     = new Plane(new Vector3(1f, 0f, 0f), -0.5f),
+            NumPoints = 4,
+            SidesType = CullMode.None,
+        };
+
+        var vestibule = new CellPhysics
+        {
+            BSP                   = EmptyLeafBsp(),
+            WorldTransform        = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
+            CellBSP               = LeafCellBsp(),
+            PortalPolygons        = new Dictionary<ushort, ResolvedPolygon> { [10] = portalPoly },
+            Portals               = new[]
+            {
+                new PortalInfo(otherCellId: (ushort)(InteriorCellId & 0xFFFFu),
+                               polygonId: 10, flags: 0),
+            },
+        };
+
+        // ── Engine + cache ────────────────────────────────────────────────
+        var engine = new PhysicsEngine();
+        engine.DataCache = new PhysicsDataCache();
+
+        // Provide a flat terrain strip at z=0 so FindEnvCollisions's outdoor
+        // fall-through has something to sample if it ever fires.
+        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(VestibuleCellId, vestibule);
+        engine.DataCache.RegisterCellStructForTest(InteriorCellId, interior);
+
+        // ── Transition ────────────────────────────────────────────────────
+        // Grounded mover, foot at world x=0.1 walking to x=0.7. The sphere
+        // (radius 0.2, center at foot + 0.2 in Z) ends with its center at
+        // world x=0.7 = interior-local x=0.4 (since interior translation is
+        // +0.3). The TallWall sits at interior-local x=0.5 with normal -X —
+        // the sphere reach (0.4 + 0.2 = 0.6) penetrates the wall by 0.1.
+        // StepUpHeight 0.04 means the mover can't scale the 5m TallWall.
+        // MakeGroundedTransition seeds Contact + OnWalkable +
+        // LastKnownContactPlane so Path 5 fires for any wall the BSP query
+        // encounters.
+        var from = new Vector3(0.1f, 0f, 0f);
+        var to   = new Vector3(0.7f, 0f, 0f);
+        var t    = BSPStepUpFixtures.MakeGroundedTransition(from, to,
+            stepUpHeight: 0.04f,
+            cellId: VestibuleCellId);
+
+        // SetCheckPos sets the candidate position FindEnvCollisions evaluates.
+        t.SpherePath.SetCheckPos(to, VestibuleCellId);
+
+        // ── Act ───────────────────────────────────────────────────────────
+        // Call FindEnvCollisions directly (now internal). Bypasses
+        // FindTransitionalPosition's sub-step iteration so we can assert on
+        // the single result.
+        var result = t.FindEnvCollisions(engine);
+
+        // ── Assert ────────────────────────────────────────────────────────
+        // Pre-A4: empty vestibule BSP returns OK, interior is never queried,
+        // result == OK (sphere walks through the wall).
+        // Post-A4: CheckOtherCells iterates the interior cell, BSPQuery on
+        // TallWall returns Slid (the wall-slide path matching B2), and
+        // FindEnvCollisions returns Slid via ApplyOtherCellResult.
+        Assert.NotEqual(TransitionState.OK, result);
+    }
+}