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>

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -1247,6 +1247,12 @@ public sealed class Transition
             this.SpherePath.WalkableAllowance = savedWalkableAllowance;
         }
 
+        // adjustedCenter (sphere slid onto polygon plane) is intentionally
+        // discarded — ValidateWalkable recomputes contact geometry from the
+        // world-space plane + foot position, consistent with the outdoor terrain
+        // path (SampleTerrainWalkable returns only plane + vertices, no adjusted
+        // sphere). The local is held only to satisfy the out param.
+
         if (!found || hitPoly is null) return false;
 
         // Transform hit polygon's plane + vertices to world space. Math is