feat(render) Campaign FW1: the degrade-level BSP gate kills the wrong punches

The offline degrade probe proved the mechanism: every Holtburg building
carries PORT nodes ONLY in its level-0 GfxObj (out to ~24-48 m); every
degraded level has zero. Retail walks the CURRENT degrade level BSP
(part->gfxobj[deg_level]) - that is what limits look-in punches to the
nearest buildings. WalkBuilding gains the degrade ladder +
SelectDrawingBsp (band pick; UpdateViewerDistance hysteresis is a port
TODO), the walk selects per viewer distance, the adapter builds
per-level BSPs, and the stab-list load rule (CLandBlock::init_buildings
@0052fd80: a full-res block loads exactly its buildings portal stab
cells) replaces load-everything in the landscape builder. The sweep now
shows clean rosters with all far-building punches gone; remaining
deltas: the near buildings 001a/0022 (50 m/28 m center distance vs the
48 m band edge - sphere-adjusted distance/hysteresis to port) and the
one ring-1 frustum boundary pair (aab50002/a9b3003c).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/Rendering/Walk/RetailFrameWalkTests.cs

@@ -50,6 +50,7 @@ public sealed class RetailFrameWalkTests
         public float ViewportHeight => 480f;
 
         public Vector3 ViewpointInBuilding(WalkBuilding building) => Vector3.Zero;
+        public float ViewerDistanceTo(WalkBuilding building) => 0f;
         public IWalkFrameContext CellContext => this;
         // Permissive near plane: every column wholly inside.
         public WalkPlane CyPlane => new(new Vector3(0, 0, 1), 0f);