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/VoxelGenerator.cpp

@@ -16,6 +16,7 @@
 #include "VoxelDensityPrimitives.h"   // spine / seal / passage — shared with the operator stack
 #include "VoxelDensityOpStack.h"      // OPSTACK Phase 1: the opt-in per-strate operator stack
 #include "VoxelHeightOp.h"            // IVoxelBiomeField — the adapter below implements it
+#include "VoxelStats.h"
 
 //=============================================================================
 // L'ADAPTATEUR DE CHAMP DE BIOMES / THE BIOME FIELD ADAPTER
@@ -3007,6 +3008,14 @@ EVoxelTileClass UVoxelGenerator::ClassifyTile(const FIntVector& OriginVoxels, in
         if (D.BridgeDensity > 0.0f || D.RidgeDensity > 0.0f) { bCanAir = false; }
 
         if (bCanSolid == bCanAir) { return EVoxelTileClass::Mixed; }
+        if (bCanSolid)
+        {
+            INC_DWORD_STAT(STAT_VoxelForgeTilesOpStackSolid);
+        }
+        else
+        {
+            INC_DWORD_STAT(STAT_VoxelForgeTilesOpStackAir);
+        }
         return bCanSolid ? EVoxelTileClass::AllSolid : EVoxelTileClass::AllAir;
     }