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/Public/VoxelDensityOpStack.h

@@ -183,6 +183,28 @@ namespace VoxelDensityOps
      *  Les six mêmes lignes apparaissent aujourd'hui dans TunnelNetwork, Maze et VerticalShafts. */
     VOXELFORGE_API TUniquePtr<IVoxelDensityOp> MakeSdfCarve(float Blend, float BaseDensity);
 
+    /** Rôle 1 — la dalle : surface de sol + surface de plafond → champ de vide. **XY-PUR** depuis
+     *  OPSTACK-DECOMPOSITION §3.1 (le terme en Z des deux bruits est parti), ce qui lui donne un
+     *  `ClassifyBox` EXACT sans échantillonnage : les deux surfaces vivent dans des bandes en Z
+     *  bornées par le contrat [-1,1] de FBM. Sert FlatPlain **et** CrystalChamber. */
+    VOXELFORGE_API TUniquePtr<IVoxelDensityOp> MakeSlabVoidSource(const FSlabGenerationParams& P, int32 Seed);
+
+    /** Rôle 3 — cylindres de hauteur infinie sur une grille monde. N'ajoute que du solide ⇒
+     *  `FillOnly` quand une colonne atteint la boîte, `Identity` (le cas courant) sinon. */
+    VOXELFORGE_API TUniquePtr<IVoxelDensityOp> MakeGridColumnMod(const FSlabGenerationParams& P, int32 Seed);
+
+    /**
+     * FlatPlain ET CrystalChamber — la même pile, **sans branchement sur le type** :
+     *   SlabVoidSource → GridColumnMod → [structural post ×3]
+     *
+     * C'est le premier vrai gain du refactor (OPSTACK-PLAN §4) : deux des huit archétypes
+     * disparaissent dans un opérateur, et leur différence redevient ce qu'elle était déjà dans
+     * `GetSlabDensity` — un jeu de valeurs par défaut, pas du code.
+     */
+    VOXELFORGE_API void BuildSlabStack(FVoxelOpStack& OutStack, const FSlabGenerationParams& P,
+                                       int32 Seed, float SpineRadius,
+                                       const UVoxelStrateManager* StrateManager);
+
     /**
      * La pile Maze complète, décomposée — PAS un `FMazeOp` monolithique :
      *   ConstantRockSource → LatticeCorridorSource → SdfRoughnessMod → SdfCarve → [structural post]