fix(physics): tighten FindWalkableSphere test assertions + header

Code review feedback on Task 2 commit 7f55e14:

- Tests 1 and 2 now assert on adjustedCenter.Z (was the wrapper's
  primary behavioral contract — sphere placed on polygon plane —
  but it was unverified). Math derived from AdjustSphereToPlane:
  iDist = (dpPos - radius) / dpMove; new center = center - movement * iDist.
- Test 2 also gains the hitPoly.Plane.Normal.Z assertion that
  Test 1 already had.
- Test 4 comment slope-angle clarification.
- BSPQuery.cs FindWalkableSphere section header now notes this is
  not a direct retail port (it wraps BSPNODE::find_walkable +
  BSPLEAF::find_walkable via the existing FindWalkableInternal).

No behavior change. Build clean; 4/4 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/BSPQueryTests.cs

@@ -304,6 +304,10 @@ public class BSPQueryTests
         Assert.Equal((ushort)0, hitPolyId);
         Assert.NotNull(hitPoly);
         Assert.Equal(1f, hitPoly!.Plane.Normal.Z, precision: 3);  // horizontal floor: normal.Z = 1
+        // AdjustSphereToPlane moves the sphere onto the plane along the movement
+        // vector. For sphere at Z=0.4, radius 0.48, downward movement -0.5, plane
+        // at Z=0: iDist = (0.4-0.48)/-0.5 = 0.16; new center.Z = 0.4 - (-0.5)*0.16 = 0.48.
+        Assert.Equal(0.48f, adjustedCenter.Z, precision: 2);
     }
 
     [Fact]
@@ -325,11 +329,14 @@ public class BSPQueryTests
             up: Vector3.UnitZ,
             out var hitPoly,
             out var hitPolyId,
-            out _);
+            out var adjustedCenter);
 
         Assert.True(found);
         Assert.Equal((ushort)1, hitPolyId);
         Assert.NotNull(hitPoly);
+        Assert.Equal(1f, hitPoly!.Plane.Normal.Z, precision: 3);  // horizontal upper floor
+        // Same math as Test 1 but offset by 3: adjustedCenter.Z = 3.0 + 0.48 = 3.48.
+        Assert.Equal(3.48f, adjustedCenter.Z, precision: 2);
     }
 
     [Fact]