fix(physics): document adjustedCenter discard + restore wall-poly test

Code review feedback on Task 3 commit 91b29d1:

- TryFindIndoorWalkablePlane: comment explaining why FindWalkableSphere's
  adjustedCenter out param is intentionally discarded (ValidateWalkable
  recomputes contact geometry from plane + foot position, consistent
  with the outdoor terrain path).
- IndoorWalkablePlaneTests: new TryFindIndoorWalkablePlane_WallPolyInBsp_ReturnsFalse
  restores integration-level coverage that the renamed NoBsp_ReturnsFalse
  lost. Verifies WalkableAllowance gate rejects a wall polygon in the
  cell BSP. Steep-poly rejection is also covered at the BSPQuery layer
  by FindWalkableSphere_SteepPoly_RejectedByWalkableAllowance.

No behavior change. Build clean; all related tests pass; same 8
pre-existing failures.

Spec: docs/superpowers/specs/2026-05-19-indoor-walkable-plane-bsp-port-design.md

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

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

@@ -167,6 +167,57 @@ public class IndoorWalkablePlaneTests
         Assert.False(found);
     }
 
+    [Fact]
+    public void TryFindIndoorWalkablePlane_WallPolyInBsp_ReturnsFalse()
+    {
+        // A polygon with a horizontal normal (Z = 0) is a wall, not a floor.
+        // walkable_hits_sphere rejects it: dp = dot(UnitZ, (0,1,0)) = 0 <= FloorZ.
+        // Regression coverage for the previous NoWalkablePolys_ReturnsFalse intent
+        // (the renamed NoBsp_ReturnsFalse only covers the null-BSP early-return).
+        Vector3[] wallVerts =
+        {
+            new Vector3(0f, 0f, 0f),
+            new Vector3(1f, 0f, 0f),
+            new Vector3(1f, 0f, 1f),
+            new Vector3(0f, 0f, 1f),
+        };
+        var resolved = new Dictionary<ushort, ResolvedPolygon>
+        {
+            [0] = new ResolvedPolygon
+            {
+                Vertices  = wallVerts,
+                Plane     = new Plane(new Vector3(0f, 1f, 0f), 0f),  // wall facing +Y
+                NumPoints = 4,
+                SidesType = CullMode.None,
+            },
+        };
+
+        var center = new Vector3(0.5f, 0f, 0.5f);
+        var bsp = BuildLeafBsp(new ushort[] { 0 }, center, 2f);
+
+        var cell = new CellPhysics
+        {
+            BSP                   = bsp,
+            WorldTransform        = Matrix4x4.Identity,
+            InverseWorldTransform = Matrix4x4.Identity,
+            Resolved              = resolved,
+        };
+
+        var transition = new Transition();
+        transition.SpherePath.WalkInterp = 1.0f;
+
+        // Foot sphere positioned to overlap the wall's plane (|Y - 0| = 0 < radius 0.48).
+        bool found = transition.TryFindIndoorWalkablePlane(
+            cell,
+            localFootCenter: new Vector3(0.5f, 0f, 0.5f),
+            sphereRadius: 0.48f,
+            out _,
+            out _,
+            out _);
+
+        Assert.False(found);
+    }
+
     [Fact]
     public void TryFindIndoorWalkablePlane_EmptyResolved_ReturnsFalse()
     {