wip(unreal): round 5 (stopped early) -- all 15 passage/strate probes walkable

Luna, round 5 goal 1 only (stopped by owner decision before the SurfaceWorld port):
- Worm and Cascading passages now pass walkability; the wide floor stays clipped while the
  narrow route core stays open.
- Hidden third failure found and fixed: long VerticalShafts diagonal legs were dropped from
  Core's spatial bins by a 100,000-bin safety cutoff (invisible to density queries);
  conservative bins restore them (18,570/18,570).
- All 15 saved probes pass a walkability-only gate; build-time walkability gate added.
NOT re-verified before the stop: W1/W8/W16 hashes, BuildHost UBT/actor run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Source/StandaloneViewer.cpp

@@ -1551,6 +1551,7 @@ namespace
         bool bNeighborhoodReadyReported=false;
         bool bShapeAuditOnly=false;
         bool bShapeCompare=false;
+        bool bWalkabilityAuditOnly=false;
         bool bFarShapeStagePending=false;
         bool bGpuFarEnabled=false;
         bool bGpuFarPipelineInitialized=false;
@@ -7889,6 +7890,7 @@ int main(int Argc,char** Argv)
         else if(Arg=="--gpu-far-audit")
         {App.bGpuFarEnabled=true;App.bGpuFarAuditEnabled=true;App.bShapeWorld=true;App.bForceGenerate=true;App.bQuitAfterClipmap=true;}
         else if(Arg=="--shape-audit")App.bShapeWorld=true,App.bShapeAuditOnly=true;
+        else if(Arg=="--walkability-audit")App.bShapeWorld=true,App.bWalkabilityAuditOnly=true;
         else if(Arg=="--grid-stair-audit")bGridStairAudit=true,App.bShapeWorld=true;
         else if(Arg=="--tile-path-hash-audit")bTilePathHashAudit=true,App.bShapeWorld=true;
         else if(Arg=="--passage-wall-detail-audit")
@@ -8059,7 +8061,7 @@ int main(int Argc,char** Argv)
         ?(StreamBenchmarkMaxLevel<0||StreamBenchmarkMaxLevel>=App.World->FullResClipLevels)
         :(App.bShapeCompare?ShapeCompareLevel>=App.World->FullResClipLevels
           :(BenchmarkLevel>=App.World->FullResClipLevels));
-    if(App.bShapeWorld&&(App.bShapeAuditOnly||bGridStairAudit||bTilePathHashAudit||App.bShapeCompare||bStreamBenchmark||BenchmarkLevel>=0)
+    if(App.bShapeWorld&&(App.bShapeAuditOnly||App.bWalkabilityAuditOnly||bGridStairAudit||bTilePathHashAudit||App.bShapeCompare||bStreamBenchmark||BenchmarkLevel>=0)
         &&!bStreamPipeline
         &&!App.BuildShapeWorldForCamera(Error,bNeedFarShapeWorld))
     {std::fprintf(stderr,"Shape interval build failed: %s\n",Error.c_str());return 2;}
@@ -8071,6 +8073,7 @@ int main(int Argc,char** Argv)
         if(!App.Snapshot)
         {std::fprintf(stderr,"Shape interval build failed: %s\n",Error.c_str());return 2;}
     }
+    if(App.bWalkabilityAuditOnly)return 0;
     if(App.bShapeAuditOnly)
     {
         if(bPassageDetailAuditOnly)