feat(world): the XY edge seal -- the fourth structural invariant

VF_ApplyBoundarySeal was purely VERTICAL: it sealed each strate's ceiling
and floor and there was no horizontal equivalent, because until the depths
were bounded the world had no sides. The GDD now says "bounded, but
gigantic", so a player who walked far enough reached unterminated
generation running off into nothing.

Built as a FORCING op, appended automatically and not author-omittable, so
an invented strate cannot forget its own edge. Because a forcing op proves
AllSolid in its band, the outer shell also becomes free to skip in tile
classification -- correctness and a perf win from one change.

  order  spine -> vertical seal -> passage carving -> XY EDGE SEAL
         -> disturbances -> diff
  A near-rim passage may pierce a vertical seal; the XY seal runs after and
  wins. A passage must never punch a hole through the world's edge.

ClassifyBox/ForcedMarginOverBox proved via the closest AABB radius with a
one-voxel margin: 8 boxes proved, 1 tile proved, 600 voxels brute-forced,
0 violations. FXYEdgeSealOp is IsXYPure. Max rim discontinuity .187 against
a 1.0 threshold -- a ramp, not a cylinder wall.

⚠️ WorldRadiusVoxels DEFAULTS TO 0, and that is deliberate. Codex flagged
that DA_Settings and BP_VoxelSettings predate the property and so do not
serialize it -- they inherit the C++ default silently. A default of 8192
would have walled in every existing world on next editor open with nothing
in the asset to explain it. 0 is a bitwise no-op (45 interior comparisons
against the unbounded path prove it). The GDD's intended ~8192 is a value
to set in the asset, deliberately, once. Documented at the declaration.

Codex found four spec errors, three of them mine: VF_ApplyBoundarySeal
lives in VoxelDensityPrimitives.h not VoxelCaveMorphology; the asset
default trap above; and my "density increases through the band" wording
contradicts the MC sign convention (solid is more NEGATIVE) -- the test
measures increasing solidness.

Build clean, 21 succeeded / 0 failed, run and verified by me.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VJiTcTSDNkKBtnNdiBtdaU

Source/VoxelForge/Private/VoxelGenerator.cpp

@@ -13,7 +13,7 @@
 #include "VoxelDiffLayer.h"
 #include "VoxelBiomeDefinition.h"
 #include "VoxelNoise.h"   // T2.a: float, SIMD-batched gradient-noise core
-#include "VoxelDensityPrimitives.h"   // spine / seal / passage — shared with the operator stack
+#include "VoxelDensityPrimitives.h"   // spine / seals / passage — shared with the operator stack
 #include "VoxelDensityOpStack.h"      // OPSTACK Phase 1: the opt-in per-strate operator stack
 #include "VoxelHeightOp.h"            // IVoxelBiomeField — the adapter below implements it
 #include "VoxelStats.h"
@@ -231,13 +231,13 @@ static float CellularNoise3D(const FVector& Position)
 //=============================================================================
 
 // Les CORPS de ces trois helpers ont déménagé dans Public/VoxelDensityPrimitives.h : la pile
-// d'opérateurs (VoxelDensityOpStack) a besoin exactement des mêmes, et deux copies de trois
-// INVARIANTS de monde (descente possible, seals qui tiennent, passages qui percent) finiraient par
+// d'opérateurs (VoxelDensityOpStack) a besoin exactement des mêmes, et deux copies des INVARIANTS
+// de monde (descente possible, seals qui tiennent, passages qui percent, bord fermé) finiraient par
 // diverger. Ces trois lignes gardent les noms locaux pour que les ~20 sites d'appel ci-dessous ne
 // bougent pas d'un caractère — le déplacement ne change AUCUN comportement.
 //
-// The BODIES moved to Public/VoxelDensityPrimitives.h; the operator stack needs the same three, and
-// two copies of three world invariants would eventually drift. These forwarders keep the local names
+// The BODIES moved to Public/VoxelDensityPrimitives.h; the operator stack needs the same primitives,
+// and duplicate world invariants would eventually drift. These forwarders keep the local names
 // so not one of the ~20 call sites below changes. No behavioural change.
 //
 // Convention INTERNE ici : positif = SOLIDE. La négation vers MC se fait sur le `return`.
@@ -558,10 +558,12 @@ UVoxelGenerator::UVoxelGenerator()
 
 void UVoxelGenerator::InitializeSettings(const UVoxelSettings* Settings)
 {
-    // Seul le seed est copié ici. Tout le reste (params de cave, transitions,
-    // blendings) vient des strate definitions via le StrateManager.
+    // Les paramètres globaux sont copiés ici une seule fois : le chemin voxel ne doit pas
+    // déréférencer l'asset de settings.
     Seed = Settings ? Settings->Seed : 0;
     OriginSpineRadius = Settings ? Settings->OriginSpineRadius : 14.0f;
+    WorldRadiusVoxels = Settings ? Settings->WorldRadiusVoxels : 0.0f;
+    EdgeSealThickness = Settings ? Settings->EdgeSealThickness : 64.0f;
 }
 
 float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) const
@@ -580,10 +582,12 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
     {
         // SOLID BEDROCK gap between two strates. The auto-carved passages still tunnel
         // through it, but the (0,0) descent stays solid here so the player digs the gap
-        // to reach the next layer. No caves, no spine, no seal — just rock + passages.
+        // to reach the next layer. No caves, no spine, no vertical seal — just rock + passages;
+        // the global XY edge seal is applied below for bounded worlds.
         float Density = 8.0f;  // bedrock solidity (positive = solid)
         const float ModSDF = StrateManager->EvaluateModifierSDF(WorldX, WorldY, WorldZ);
         ApplyPassageCarving(Density, ModSDF, 8.0f, 0.0f);
+        VF_ApplyXYEdgeSeal(Density, WorldX, WorldY, WorldRadiusVoxels, EdgeSealThickness, 8.0f);
         Result = -Density;
     }
     else if (StrateManager)
@@ -687,6 +691,8 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
                 OpCtx.ChunkCoord    = ChunkCoord;
                 OpCtx.Seed          = (uint32)Seed;
                 OpCtx.LayoutVersion = LayoutVersion;
+                OpCtx.WorldRadiusVoxels = WorldRadiusVoxels;
+                OpCtx.EdgeSealThickness = EdgeSealThickness;
 
                 // ⚠️ LE MAPPING VIT DANS `VF_BuildOpStackForChunk` (haut de ce fichier) ET NULLE
                 // PART AILLEURS — `ClassifyTile` appelle la MÊME fabrique. Un verdict de tuile issu
@@ -807,6 +813,12 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
         Result = GetDensityWithParams(WorldX, WorldY, WorldZ, FallbackParams, FallbackFP, 0);
     }
 
+    // The edge is the final generated structural invariant. Passage carving is ordered before it
+    // in every archetype/stack, and this MC-facing pass also covers out-of-layout air plus any
+    // disturbance that might otherwise carve back into the shell. Player edits remain separate
+    // below and are deliberately still the user override path.
+    VF_ApplyXYEdgeSealMC(Result, WorldX, WorldY, WorldRadiusVoxels, EdgeSealThickness, 8.0f);
+
     //=========================================================================
     // PLAYER MODIFICATIONS (diff layer)
     //=========================================================================
@@ -873,7 +885,8 @@ float UVoxelGenerator::GetDensityWithParams(float WorldX, float WorldY, float Wo
     //   4h. Pinch/Bottleneck: ellipsoidal passage narrowing for chokepoints
     //   5.  Worm tunnels: additional organic connectivity
     //   6.  Boundary seal: solid rock at strate top/bottom
-    //   7.  Modifiers: passages between strates (punch through seals)
+    //   7.  Modifiers: passages between strates (punch through vertical seals)
+    //   8.  XY edge seal: global bounded-world shell wins over passages at the rim
     //
     // Convention: positive density = solid, negative = air (internally).
     // At the end, we negate for the MC table (negative = solid there).
@@ -1945,6 +1958,10 @@ float UVoxelGenerator::GetDensityWithParams(float WorldX, float WorldY, float Wo
         ApplyPassageCarving(Density, ModSDF, Params.BaseDensity, Params.BoundarySealThickness);
     }
 
+    // Fourth structural post: the XY edge wins over a passage near the rim.
+    VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
+        WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
+
     // Convention MC: négatif = solide, positif = air.
     // La logique interne utilise positif = solide (plus lisible), donc on négate.
     return -Density;
@@ -2146,7 +2163,7 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
     }
 
     //=========================================================================
-    // STEP 5: BOUNDARY SEAL + STEP 6: PASSAGES
+    // STEP 5: BOUNDARY SEAL + STEP 6: PASSAGES + STEP 7: XY EDGE SEAL
     //=========================================================================
     // Même logique que TunnelNetwork — factorisée dans les helpers ci-dessus.
     ApplyOriginSpine(Density, WorldX, WorldY, WorldZ,
@@ -2163,6 +2180,9 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
         ApplyPassageCarving(Density, ModSDF, Params.BaseDensity, Params.BoundarySealThickness);
     }
 
+    VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
+        WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
+
     // Convention MC: négatif = solide.
     return -Density;
 }
@@ -2276,6 +2296,9 @@ float UVoxelGenerator::GetMazeDensity(float WorldX, float WorldY, float WorldZ,
         ApplyPassageCarving(Density, ModSDF, Params.BaseDensity, Params.BoundarySealThickness);
     }
 
+    VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
+        WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
+
     return -Density;
 }
 
@@ -2536,6 +2559,9 @@ float UVoxelGenerator::SurfaceDensityFromColumn(float WorldX, float WorldY, floa
         ApplyPassageCarving(Density, ModSDF, S.BaseDensity, S.BoundarySealThickness);
     }
 
+    VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
+        WorldRadiusVoxels, EdgeSealThickness, S.BaseDensity);
+
     return -Density;
 }
 
@@ -2681,8 +2707,6 @@ bool UVoxelGenerator::GetSurfaceHeightAt(float WorldX, float WorldY, int32 Chunk
 
 EVoxelTileClass UVoxelGenerator::ClassifyTile(const FIntVector& OriginVoxels, int32 Step, int32 CellsPerAxis) const
 {
-    if (!StrateManager) return EVoxelTileClass::Mixed;
-
     // Mêmes clamps que GenerateMesh — le verdict doit couvrir le treillis réellement échantillonné.
     Step = FMath::Max(1, Step);
     const int32 CPA     = FMath::Clamp(CellsPerAxis, 2, CHUNK_SIZE);
@@ -2708,6 +2732,20 @@ EVoxelTileClass UVoxelGenerator::ClassifyTile(const FIntVector& OriginVoxels, in
         if (DiffLayer->HasAnyModInChunkRange(MinChunk, MaxChunk)) return EVoxelTileClass::Mixed;
     }
 
+    // The bounded shell is global: it also covers gaps and out-of-layout air, so prove it before
+    // consulting the strate layout. The final MC edge pass uses a positive base, hence 1.0 is a
+    // deliberately conservative proof input here; the stack op performs the same proof with the
+    // archetype base for its own sub-box.
+    const FBox TileVoxelBox(FVector((float)MinX, (float)MinY, (float)MinZ),
+                            FVector((float)MaxX, (float)MaxY, (float)MaxZ));
+    if (VF_XYEdgeSealBoxIsForcedSolid(TileVoxelBox, WorldRadiusVoxels,
+                                      EdgeSealThickness, 1.0f))
+    {
+        return EVoxelTileClass::AllSolid;
+    }
+
+    if (!StrateManager) return EVoxelTileClass::Mixed;
+
     bool bCanSolid = true;   // "tout le treillis est solide" encore prouvable
     bool bCanAir   = true;   // "tout le treillis est air" encore prouvable
 
@@ -3140,6 +3178,8 @@ EVoxelTileClass UVoxelGenerator::ClassifyTile(const FIntVector& OriginVoxels, in
         OpCtx.Seed          = (uint32)Seed;
         OpCtx.LayoutVersion = TC_LayoutVersion;
         OpCtx.Step          = Step;
+        OpCtx.WorldRadiusVoxels = WorldRadiusVoxels;
+        OpCtx.EdgeSealThickness = EdgeSealThickness;
 
         FVoxelOpStack TileStack;
         if (!VF_BuildOpStackForChunk(CaveType, Refs, Seed, OriginSpineRadius,
@@ -3940,6 +3980,9 @@ float UVoxelGenerator::GetVerticalShaftDensity(float WorldX, float WorldY, float
         ApplyPassageCarving(Density, ModSDF, Params.BaseDensity, Params.BoundarySealThickness);
     }
 
+    VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
+        WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
+
     return -Density;
 }
 
@@ -4097,5 +4140,8 @@ float UVoxelGenerator::GetFloatingIslandDensity(float WorldX, float WorldY, floa
         ApplyPassageCarving(Density, ModSDF, Params.BaseDensity, Params.BoundarySealThickness);
     }
 
+    VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
+        WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
+
     return -Density;
 }