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/VoxelStrateManager.cpp

@@ -571,8 +571,10 @@ bool UVoxelStrateManager::UsesOperatorStackForChunk(const FIntVector& ChunkCoord
     // and falls back to the switch, so the box can be ticked anywhere without breaking anything.
     switch (Def->GeneratorType)
     {
-    case ECaveGeneratorType::Maze:  return true;
-    default:                        return false;
+    case ECaveGeneratorType::Maze:            return true;   // Phase 1
+    case ECaveGeneratorType::FlatPlain:                      // Phase 2 — les deux partagent
+    case ECaveGeneratorType::CrystalChamber:  return true;   //   UNE seule pile (BuildSlabStack)
+    default:                                  return false;
     }
 }