fix(physics): Phase 2 — pass foot-sphere center to ResolveCellId

Visual test of Phase 2 portal traversal showed walls still didn't
block from inside buildings. Diagnosis: ResolveCellId was being
called with sp.CheckPos (entity reference, at the feet — world
Z=terrain) instead of sp.GlobalSphere[0].Origin (foot sphere center,
~0.5m above terrain). Combined with the +0.02f Z-bump on cached
cell origins (for render z-fight prevention), the test position
landed at cell-local Z=-0.02 — just below the cell floor — and
PointInsideCellBsp correctly reported "outside" for every cell.
CheckBuildingTransit never added candidates; player CellId stayed
outdoor; indoor cell-BSP collision branch never fired; walls didn't
block.

Retail's check_building_transit uses sphere.Center (the sphere CENTER,
not the entity reference) per the pseudocode at
docs/research/acclient_indoor_transitions_pseudocode.md:222-238.

Three call sites updated (PhysicsEngine x2 inside ResolveWithTransition;
TransitionTypes inside Transition.FindEnvCollisions).

Also adds a [check-bldg] diagnostic line to CheckBuildingTransit (gated
on the existing ACDREAM_PROBE_INDOOR_BSP flag) so future verification
captures show per-portal inside/outside results without needing
another diagnostic flag.

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

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

src/AcDream.Core/Physics/CellTransit.cs

@@ -186,11 +186,28 @@ public static class CellTransit
         foreach (var portal in building.Portals)
         {
             var otherCell = cache.GetCellStruct(portal.OtherCellId);
-            if (otherCell?.CellBSP?.Root is null) continue;
+            if (otherCell?.CellBSP?.Root is null)
+            {
+                if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
+                {
+                    string reason = otherCell is null ? "cell not cached" : "CellBSP null";
+                    Console.WriteLine(System.FormattableString.Invariant(
+                        $"[check-bldg] portal->0x{portal.OtherCellId:X8} skipped: {reason}"));
+                }
+                continue;
+            }
 
             // Sphere center in the OTHER cell's local space.
             var localCenter = Vector3.Transform(worldSphereCenter, otherCell.InverseWorldTransform);
-            if (BSPQuery.PointInsideCellBsp(otherCell.CellBSP.Root, localCenter))
+            bool inside = BSPQuery.PointInsideCellBsp(otherCell.CellBSP.Root, localCenter);
+
+            if (PhysicsDiagnostics.ProbeIndoorBspEnabled)
+            {
+                Console.WriteLine(System.FormattableString.Invariant(
+                    $"[check-bldg] portal->0x{portal.OtherCellId:X8} wpos=({worldSphereCenter.X:F3},{worldSphereCenter.Y:F3},{worldSphereCenter.Z:F3}) lpos=({localCenter.X:F3},{localCenter.Y:F3},{localCenter.Z:F3}) inside={inside}"));
+            }
+
+            if (inside)
             {
                 candidates.Add(portal.OtherCellId);
             }

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -769,7 +769,7 @@ public sealed class PhysicsEngine
 
             return new ResolveResult(
                 sp.CheckPos,
-                ResolveCellId(sp.CheckPos, sphereRadius, sp.CheckCellId),
+                ResolveCellId(sp.GlobalSphere[0].Origin, sphereRadius, sp.CheckCellId),
                 onGround,
                 collisionNormalValid,
                 collisionNormal);
@@ -787,7 +787,7 @@ public sealed class PhysicsEngine
         uint partialCellId = sp.CheckCellId != 0 ? sp.CheckCellId : cellId;
         return new ResolveResult(
             sp.CheckPos,
-            ResolveCellId(sp.CheckPos, sphereRadius, partialCellId),
+            ResolveCellId(sp.GlobalSphere[0].Origin, sphereRadius, partialCellId),
             partialOnGround,
             collisionNormalValid,
             collisionNormal);

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -1181,7 +1181,7 @@ public sealed class Transition
         Vector3 footCenter = sp.GlobalSphere[0].Origin;
         float   sphereRadius = sp.GlobalSphere[0].Radius;
 
-        uint resolvedOutdoorCellId = engine.ResolveCellId(sp.CheckPos, sphereRadius, sp.CheckCellId);
+        uint resolvedOutdoorCellId = engine.ResolveCellId(sp.GlobalSphere[0].Origin, sphereRadius, sp.CheckCellId);
         if (resolvedOutdoorCellId != sp.CheckCellId)
             sp.SetCheckPos(sp.CheckPos, resolvedOutdoorCellId);