feat(stats): stat VoxelForge -- make tile skipping and the column memo observable

The plugin had ZERO stat counters, so every claim this refactor makes was
harness-only: "skipped correctly" and "skipped nothing" render identically.
CODEX-TASK-001 and -002, executed by Codex (gpt-5.6-luna xhigh), reviewed
against the source.

Eight DWORD counters in a new stat group:
  GenerateTileResult -- TilesClassified / SkippedAllSolid / SkippedAllAir / Meshed
  ClassifyTile bAnyCave exit -- TilesOpStackSolid / TilesOpStackAir
  FSurfaceColumnSource::GetColumn -- ColumnMemoHit / ColumnMemoMiss

The op-stack counters are separate from the lumped skip counters on purpose:
ClassifyTile also proves AllSolid on its hand-written bedrock-gap path with no
strate opted in, so the lumped number cannot show a before/after. The op-stack
pair is zero BY CONSTRUCTION until a strate ticks the flag.

GetColumn is instrumented and deliberately NOT fixed -- the instrument and the
fix in one build would make each other unreadable.

Verified against UE_5.7 Stats.h rather than assumed (first stats use in this
plugin, no in-repo precedent): all four macro arities, and INC_DWORD_STAT ->
FThreadStats::AddMessage, so the worker-thread safety both sites need holds by
mechanism. Not built -- Jahni builds.

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

Source/VoxelForge/Private/VoxelDensityOpStack.cpp

@@ -29,6 +29,7 @@
 #include "VoxelTerrainOpDefinition.h" // ApplyTo — l'override d'op PAR SALLE (étape C1)
 #include "VoxelStrateManager.h"       // EvaluateModifierSDF / AnyPassageNearBox
 #include "VoxelTypes.h"               // SmoothStep01, VOXEL_NOISE_SCALE
+#include "VoxelStats.h"
 
 #include <atomic>                     // l'id d'instance non recyclé du mémo de colonne
 
@@ -641,6 +642,7 @@ namespace
             FSlot& S = Slots[Idx];
             if (S.Key != ColumnKey || S.X != WorldX || S.Y != WorldY)
             {
+                INC_DWORD_STAT(STAT_VoxelForgeColumnMemoMiss);
                 S.Key = ColumnKey;  S.X = WorldX;  S.Y = WorldY;
                 FColumn& C = S.C;
 
@@ -700,6 +702,10 @@ namespace
                     if (Slope > KINDA_SMALL_NUMBER) { C.DirX = GX / Slope; C.DirY = GY / Slope; }
                 }
             }
+            else
+            {
+                INC_DWORD_STAT(STAT_VoxelForgeColumnMemoHit);
+            }
             return S.C;
         }