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>

src/AcDream.App/Rendering/Walk/RetailFrameWalk.cs

@@ -136,6 +136,11 @@ public sealed class RetailFrameWalk
     {
         sink.Emit(WalkEvent.Building(building.PositionCellId));
         if (!building.HasGeometry) return;
+        // Retail walks the CURRENT degrade level's drawing BSP
+        // (part->gfxobj[deg_level]); a degraded-out slot skips everything
+        // after publishing the portal list.
+        WalkBspNode? bsp = building.SelectDrawingBsp(ctx.ViewerDistanceTo(building));
+        if (bsp is null) return;
 
         int viewCount = Math.Max(activeViews.ViewCount, 0);
         var passSink = new PortalPassSink(sink);
@@ -144,11 +149,11 @@ public sealed class RetailFrameWalk
         {
             ctx.SetActiveView(activeViews, v);
             WalkBuildingPortals.BuildDrawPortalsOnly(
-                building.DrawingBsp, 1, viewpoint,
+                bsp, 1, viewpoint,
                 (portalRef, pass) => WalkBuildingPortals.DrawPortal(
                     _outdoorPView, building, portalRef, pass, ctx, passSink));
             WalkBuildingPortals.BuildDrawPortalsOnly(
-                building.DrawingBsp, 2, viewpoint,
+                bsp, 2, viewpoint,
                 (portalRef, pass) => WalkBuildingPortals.DrawPortal(
                     _outdoorPView, building, portalRef, pass, ctx, passSink));
         }

src/AcDream.App/Rendering/Walk/WalkBuilding.cs

@@ -37,6 +37,10 @@ public sealed class WalkBspNode
     public bool IsPortal => InPortals is not null;
 }
 
+/// <summary>One degrade-ladder level: the drawing BSP of that level's
+/// GfxObj (portal-only view) and the level's far distance bound.</summary>
+public readonly record struct WalkBuildingDegradeLevel(float MaxDist, WalkBspNode? DrawingBsp);
+
 /// <summary>The walk's building model (retail <c>CBuildingObj</c> +
 /// <c>BuildInfo</c> as the frame walk consumes them).</summary>
 public sealed class WalkBuilding
@@ -47,12 +51,35 @@ public sealed class WalkBuilding
 
     public WalkBldPortal[] Portals = [];
 
-    /// <summary>The drawing BSP of part 0's GfxObj (portal-only view).</summary>
+    /// <summary>The drawing BSP of part 0's BASE GfxObj (portal-only view).
+    /// Used directly when the model has no degrade ladder.</summary>
     public WalkBspNode? DrawingBsp;
 
+    /// <summary>The degrade ladder (near→far). Retail walks the CURRENT
+    /// level's drawing BSP (<c>part->gfxobj[deg_level]</c>) — and building
+    /// degrade models beyond the first band carry NO portal nodes, which is
+    /// what limits look-in punches to nearby buildings (probed 2026-08-30:
+    /// every Holtburg building has ports at level 0 only).</summary>
+    public WalkBuildingDegradeLevel[] DegradeLevels = [];
+
     /// <summary><c>part->gfxobj[deg_level] != 0</c> — a degraded-out slot
     /// skips the whole building AFTER publishing the portal list.</summary>
     public bool HasGeometry = true;
+
+    /// <summary>Level selection by viewer distance. Simplified band pick
+    /// (first level whose MaxDist covers the distance) — the faithful
+    /// hysteresis of <c>CPhysicsPart::UpdateViewerDistance</c> is a port
+    /// TODO; still fixtures are insensitive to hysteresis.</summary>
+    public WalkBspNode? SelectDrawingBsp(float viewerDistance)
+    {
+        if (DegradeLevels.Length == 0) return DrawingBsp;
+        foreach (WalkBuildingDegradeLevel level in DegradeLevels)
+        {
+            if (viewerDistance <= level.MaxDist)
+                return level.DrawingBsp;
+        }
+        return null;   // degraded out entirely
+    }
 }
 
 /// <summary>
@@ -246,6 +273,11 @@ public interface IWalkBuildingFrameContext
 {
     Vector3 ViewpointInBuilding(WalkBuilding building);
 
+    /// <summary>The viewer's distance to the building
+    /// (<c>CPhysicsPart::UpdateViewerDistance</c>'s input) — selects the
+    /// degrade level whose drawing BSP the portal pass walks.</summary>
+    float ViewerDistanceTo(WalkBuilding building);
+
     /// <summary>Project + clip one building-local portal polygon against
     /// the ACTIVE view (retail: GetClip with do_clip=1 in the building
     /// frame). Returns the surviving count.</summary>