feat: Phase 2 first port — FlatPlain + CrystalChamber collapse into one op

Jahni closed OPSTACK-DECOMPOSITION 3.1: the slab noise Z term was not
intentional character. Phase 1 also closed — the visual A/B on Maze passed.

Two changes, deliberately together, kept attributable by the test:

1. Design: GetSlabDensity's floor and ceiling noise lose their Z terms. A floor
   height no longer depends on the altitude you sample it from. The ceiling keeps
   its + 3000.0f, which is a decorrelation offset, not a Z term. The world
   re-tunes once — a different slice of the noise field, not a worse one.

2. Refactor: the now-XY-pure function ports to FSlabVoidSource + FGridColumnMod
   plus the three structural ops. BuildSlabStack has NO branch on archetype
   because GetSlabDensity never had one — CrystalChamber is FlatPlain with a
   bigger CeilingRoughness. 8 archetypes -> 7.

SlabEquivalence compares against the reference AS IT IS NOW and runs the whole
battery on both slots, so green means the port is a pure refactor and any visual
delta is attributable to the Z-term removal alone. The attribution comes from the
test, not from splitting it across two builds.

The payoff 3.1 was actually about: FSlabVoidSource::ClassifyBox is exact and needs
no sampling. FBM is contractually [-1,1], so both surfaces live in Z bands with
known bounds — a tile below the floor band is provably solid, a tile between the
bands provably air. ClassifyTile proves zero tiles for these archetypes today.
FGridColumnMod answers Identity when no column reaches the box, which is what lets
the source's AllAir verdict survive the fold.

UNVERIFIED: not compiled.

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

Source/VoxelForge/Private/VoxelGenerator.cpp

@@ -562,6 +562,22 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
                     VoxelDensityOps::BuildMazeStack(CP_OpStack, CP_Maze, Seed,
                                                     OriginSpineRadius, StrateManager);
                     break;
+
+                case ECaveGeneratorType::FlatPlain:
+                case ECaveGeneratorType::CrystalChamber:
+                    // UN SEUL cas pour les deux, comme le `switch` de production juste en dessous :
+                    // `GetSlabDensity` ne les distingue pas non plus. Voir BuildSlabStack.
+                    // Même garde de strate dégénérée : GetSlabDensity court-circuite sur `1.0f`.
+                    if (CP_Slab.StrateTopWorldZ - CP_Slab.StrateBottomWorldZ <= 0.0f)
+                    {
+                        CP_UseOpStack = false;
+                        break;
+                    }
+                    OpCtx.StrateTopWorldZ    = CP_Slab.StrateTopWorldZ;
+                    OpCtx.StrateBottomWorldZ = CP_Slab.StrateBottomWorldZ;
+                    VoxelDensityOps::BuildSlabStack(CP_OpStack, CP_Slab, Seed,
+                                                    OriginSpineRadius, StrateManager);
+                    break;
                 default:
                     // UsesOperatorStackForChunk ne rend true que pour les archétypes portés, donc
                     // on ne devrait jamais arriver ici. Si ça arrive, retomber sur le `switch`
@@ -1807,8 +1823,17 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
     // Signed noise allows both hills (noise > 0 → floor rises) and
     // valleys (noise < 0 → floor dips) for natural rolling ground.
     //
-    // Z frequency is set very low (5% of XY) so the floor features are
-    // broad and horizontal — like natural geological ground, not bumpy walls.
+    // ⚠️ XY-PUR / XY-PURE (OPSTACK-DECOMPOSITION §3.1, tranché par Jahni 2026-07-27).
+    // La 3e coordonnée était `WorldZ * FF * 0.05f` : une hauteur de sol qui dépendait de
+    // l'altitude d'où on la demandait. Le coefficient était minuscule, donc ça se lisait comme un
+    // léger étirement vertical plutôt que comme un bug — mais ça bloquait le cache de colonnes T1.a
+    // et rendait toute classification de boîte inexacte. Constante ⇒ la surface est une vraie
+    // fonction de (X,Y). Le monde se re-tune une fois : on échantillonne une autre tranche du champ
+    // de bruit, donc la forme du sol change (elle ne se dégrade pas).
+    //
+    // The 3rd coord was WorldZ * FF * 0.05f — a floor height that depended on the altitude you
+    // asked from. Now a constant, so the surface is a genuine function of (X,Y): the T1.a column
+    // cache and an exact box verdict both become available. Worlds re-tune once.
 
     const float FloorZ = Params.StrateBottomWorldZ + StrateHeight * Params.FloorRelativeHeight;
 
@@ -1819,7 +1844,7 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
         FloorNoise = FractalNoise3D(FVector(
             WorldX * FF + SeedF * 7.3f,
             WorldY * FF + SeedF * 11.1f,
-            WorldZ * FF * 0.05f           // Very low Z freq → horizontal ground features
+            0.0f                          // XY-pur : plus aucune dépendance en Z / no Z dependence
         ), VoxelGenLOD::Eff(3)) * VOXEL_NOISE_SCALE * Params.FloorRoughness;
     }
 
@@ -1836,9 +1861,10 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
     // This asymmetry (only downward protrusions, never upward pockets) creates
     // the crystal-forest / stalactite silhouette from below.
     //
-    // Z frequency is also low so formations have horizontal extent — each
-    // "crystal" or "stalactite" is wide and sweeps across the ceiling, not
-    // a sharp spike (use high frequency for spike-like features if desired).
+    // XY-PUR, même raison que le sol ci-dessus (§3.1). Le `+ 3000.0f` RESTE : ce n'est pas un
+    // terme en Z, c'est le décalage qui décorrèle le champ du plafond de celui du sol.
+    // XY-pure for the same reason as the floor. The + 3000.0f STAYS — it is not a Z term, it is
+    // the offset that decorrelates the ceiling's noise field from the floor's.
 
     const float CeilZ = Params.StrateBottomWorldZ + StrateHeight * Params.CeilingRelativeHeight;
 
@@ -1849,7 +1875,7 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
         float RawNoise = FractalNoise3D(FVector(
             WorldX * CF + SeedF * 17.3f + 1000.0f,
             WorldY * CF + SeedF * 19.7f + 2000.0f,
-            WorldZ * CF * 0.08f + 3000.0f   // Low Z freq → formations extend horizontally
+            3000.0f                         // XY-pur : décalage de décorrélation seul / offset only
         ), VoxelGenLOD::Eff(3)) * VOXEL_NOISE_SCALE;
 
         // abs() → formations ONLY hang down, never push ceiling up into solid rock.