wip(assembler): round S4 -- continuous far ceiling at the strate's upper boundary

S3's far cap sat up to 8.2 m above the near roof and read as floating pieces.
The far render cap now projects onto the owning strate's upper boundary
(density unchanged), on a global XY lattice, LOD4 16-voxel grid + LOD5-8
128-voxel. Cap agreement 0.05 cm (100 cm control detected), 2520/2520 upward
rays hit roof on the owner walk. Far recipe v2; LOD0-3 hashes equal S2/S3.
Tile p95 flat; cold clipmap +4.4%. Far-cliff fluting left open (coarse MC step).

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

Source/VoxelForge/Private/VoxelWorld.cpp

@@ -24,6 +24,7 @@
 #include "VoxelClipmapDesiredTiles.h"
 #include "VoxelCollisionDesiredTiles.h"
 #include "VoxelPreEditDensityCache.h"
+#include "RealtimeMeshLibrary.h"
 // IWYU (FPSemantics = Precise ⇒ plus de PCH partagé) : GetPlayerPosition déréférence le pawn, donc
 // APawn doit être COMPLET — `Casts.h` n'en donne qu'une déclaration avant. APlayerController n'est
 // complet que par transitivité : on l'inclut explicitement.
@@ -12471,21 +12472,30 @@ bool AVoxelWorld::ApplyMeshToTile(FChunkResult& Result)
     // (default config already maps material slot = polygroup index); only config sections that
     // exist — the bHas* flags come from the worker. Collision at level 0 only (T1.c), both groups.
     const uint64 CollisionConfigStartCycles = bProfileApply ? FPlatformTime::Cycles64() : 0;
+    const int32 NumIndices = Result.NumTriangles * 3;
+    const int32 FirstCapIndex = FMath::Clamp(
+        NumIndices - Result.NumCeilingTriangles * 3, 0, NumIndices);
     if (bHasGroundTris)
     {
         FRealtimeMeshSectionConfig GroundConfig(0);
         GroundConfig.bCastsShadow = bCastShadow;
-        RTMesh->UpdateSectionConfig(
+        const FRealtimeMeshStreamRange GroundRange =
+            URealtimeMeshBlueprintFunctionLibrary::MakeStreamRange(
+                0, Result.NumVertices, 0, FirstCapIndex);
+        RTMesh->CreateSection(
             FRealtimeMeshSectionKey::CreateForPolyGroup(GroupKey, 0),
-            GroundConfig, /*bShouldCreateCollision*/ bWantsCollision);
+            GroundConfig, GroundRange, /*bShouldCreateCollision*/ bWantsCollision);
     }
     if (bHasCeilingTris)
     {
         FRealtimeMeshSectionConfig CapConfig(1);
         CapConfig.bCastsShadow = false;                      // the cap never casts (see above)
-        RTMesh->UpdateSectionConfig(
+        const FRealtimeMeshStreamRange CapRange =
+            URealtimeMeshBlueprintFunctionLibrary::MakeStreamRange(
+                0, Result.NumVertices, FirstCapIndex, NumIndices);
+        RTMesh->CreateSection(
             FRealtimeMeshSectionKey::CreateForPolyGroup(GroupKey, 1),
-            CapConfig, /*bShouldCreateCollision*/ bWantsCollision);
+            CapConfig, CapRange, /*bShouldCreateCollision*/ bWantsCollision);
     }
     const double CollisionConfigSeconds = bProfileApply
         ? FPlatformTime::ToSeconds64(FPlatformTime::Cycles64() - CollisionConfigStartCycles)