feat(opstack B4): port Columns (4d), Domes (4g), Pinch (4h), FloorBias + Column op in the test pool

STAGE B group 4 of 5 -- and with it, ALL TWELVE detail modifiers are ported. 19 ops:
  ConstantRock -> RoomGraph -> SdfCarve -> Roughness -> Terrace -> LayerLines -> Ribbing
  -> Overhang -> Cliff -> Scallop -> Arch -> RoomColumn -> Dome -> Pinch -> FloorBias
  -> Worms -> [structural x3]
Still NOT wired: UsesOperatorStackForChunk returns false for TunnelNetwork. What is missing is C1,
the per-room op override -- which adds NO operator, it changes what eleven of them READ.

TRANSCRIBED
- FRoomColumnMod (4d): walks SDFCache.Columns. Not FGridColumnMod -- that one rolls cylinders on a
  world grid for FlatPlain/CrystalChamber; this one iterates a per-room bake.
- FDomeMod (4g): upward half-ellipsoid, up to 2 per room, capped at 0.85 * room radius.
- FPinchMod (4h): perimeter-anchored ellipsoid with the SideFactor term that leaves the passage
  axis clear.
- FFloorBiasMod: quadratic fill below room centre, only in definite air. LAST in the chain and not
  interchangeable -- it exists to undo what roughness (4b) did to floors. That is why the operator
  order in BuildTunnelNetworkStack is the original's order line for line.

THE TRAP IN THIS GROUP, WHICH IS THE PIT LESSON A THIRD TIME
Columns have NO strate parameter. The per-voxel loop has no `if (ColumnDensity > 0)`, and the bake
reads OpParams (a FRESH struct with only the room's op applied), so FStrateGenerationParams::
ColumnDensity is read by nothing. Consequences, all three of which shaped this commit:
  1. The `ColumnDensity = 0` line in the old DisableStageBModifiers was inert. It was never what
     kept columns off; the absence of a Column op in the pool was.
  2. No FeatureProbes entry can cover columns -- there is no param to switch off. And a second
     stack built on a pool without the Column op would SHARE the thread_local SDF cache (the pool
     is not in its key) and return identical densities, i.e. it would lie. Exactly the trap that
     was documented for pits.
  3. So the stage-A guard that ERRORED on TotalColumns > 0 is inverted in this same commit: it now
     REQUIRES TotalColumns > 0. Asking the bake what it baked is the only sound check here.

DisableStageBModifiers is deleted rather than left empty: its emptiness is the measure of stage B
being complete, and an empty function still being called is how a step gets forgotten.

TEST (same commit): op count 15 -> 19; MakeShaftOpPool renamed MakeRoomOpPool and given a third
entry (Column, Probability 1.0 on all three so every room draws one of the three evenly); three
more param probes (dome/pinch/floor bias); the column guard inverted.

⚠️ THE POOL MUST STAY Pit/Chimney/Column UNTIL C1. Eleven of the twelve modifiers read the per-room
param copy in the original and the strate params here; those agree only while no room carries a
DETAIL-type op, and ApplyTo(Pit/Chimney/Column) writes only pit/chimney/column fields. A Terrace
entry would break the equivalence -- which is precisely the test C1 adds, and the only possible
proof that the override works.

IF THIS GROUP IS WRONG: dome/pinch/arch diffs land inside open cave near room centres and
perimeters; a column error shows up as a ring of diffs at fixed XY through the whole room height.

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

Source/VoxelForge/Public/VoxelDensityOpStack.h

@@ -266,14 +266,15 @@ namespace VoxelDensityOps
                                                 const UVoxelStrateManager* StrateManager);
 
     /**
-     * TunnelNetwork — **ÉTAPES A + B1 + B2 + B3, PILE ENCORE INCOMPLÈTE** :
+     * TunnelNetwork — **ÉTAPES A + B COMPLÈTES, 19 ops** :
      *   ConstantRock → RoomGraph(warp + pits + cheminées) → SdfCarve → CaveRoughness(4b)
-     *   → Terrace → LayerLines → Ribbing → Overhang → Cliff → Scallop → Arch
-     *   → Worms → [structural ×3]
+     *   → Terrace → LayerLines → Ribbing → Overhang → Cliff → Scallop → Arch → RoomColumn(4d)
+     *   → Dome(4g) → Pinch(4h) → FloorBias → Worms → [structural ×3]
      *
-     * ⛔ NE PAS brancher cet archétype dans `UsesOperatorStackForChunk` avant l'étape C : les quatre
-     * modificateurs de détail restants (4c–4h) et l'override d'op par salle ne sont pas portés, donc
-     * le monde y perdrait du détail. Le test compare avec ces amplitudes à zéro.
+     * ⛔ NE PAS brancher cet archétype dans `UsesOperatorStackForChunk` avant l'étape C :
+     * l'override d'op PAR SALLE n'est pas porté, donc les onze modificateurs qui devraient lire les
+     * params de la salle la plus proche lisent ceux de la strate. Identique tant qu'aucune salle ne
+     * porte un op de type détail — ce que le test garantit et qu'un monde réel ne garantit pas.
      *
      * ⚠️ `FRoomGraphSource` **APPELLE** `BuildChunkCache`/`EvaluateSDFCached`, il ne les transcrit
      * pas : c'est là que vit la discipline d'invariance de fenêtre à deux régions (`ARCHITECTURE