fix(world): drain workers before mutating strate layout/passages (VF-01)

Initialize does StrateLayout.Empty() and Passages.Empty()/Add() -- freeing and
reallocating both -- with no guard, while mesher workers read them through
AnyPassageNearBox, EvaluateModifierSDF and FindSlotIndexForChunkZ. The epoch is
bumped AFTER, so previous-epoch workers are live during the mutation; the epoch
rejects a finished result, it cannot make a read of a freed allocation safe.
Same class as the DiffLayer.ChunkMods carve-vs-stream AV that ModsLock fixed.

Chose the drain over the two alternatives:
  - FRWLock on the arrays: correct, exact in-repo precedent, but a read lock on
    the per-voxel hot path (~43k/tile) would contaminate the perf measurement
    CODEX-TASK-001/002 are queued to take. Worst possible timing.
  - Immutable generation snapshot (Sol's proposal): right long-term, refactors
    the whole StrateManager API surface. A design conversation, not an agent's
    unprompted call.
  - Drain: zero hot-path cost, reuses the machinery EndPlay already proved, and
    its stall lands only on human-initiated editor actions, never during play.

FScopedGenerationPause raises a new bGenerationPaused (deliberately NOT
bShuttingDown, which means teardown) and drains both reader populations: chunk
tasks via ActiveTaskCount and decoration tasks via a new
WaitForDecorationTasks, whose refactor also removed NotifyShutdown's duplicate
spin loop. On timeout it does NOT mutate -- logs an Error and leaves the world
consistent. A timeout that proceeds is what the bug already does.

RebuildStrates, OnObjectModifiedInEditor and ChangeSeed cannot reach Initialize
without the pause. BeginPlay untouched (no tasks yet). EndPlay untouched --
VF-02's 3s timeout is a separate deliberate decision.

Verified: four files, no density/mesher file, so terrain cannot move.
ShouldAbortWork replaced three existing checks, adding none. No worker path
waits on the game thread, so the drain is bounded.

Not built -- Jahni builds.

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

Source/VoxelForge/Public/VoxelWorld.h

@@ -25,6 +25,7 @@ class UMaterialParameterCollection;
 class UVolumeTexture;
 class UMaterialInterface;
 class UMaterialInstanceDynamic;
+class FScopedGenerationPause;
 namespace RealtimeMesh { struct FRealtimeMeshStreamSet; }   // T1.f — worker-built geometry buffers
 
 /**
@@ -373,6 +374,8 @@ public:
     UVolumeTexture* GetDensityVolumeTexture(int32 Level = 0) const;
 
 private:
+    friend class FScopedGenerationPause;
+
     /** Get/create the shared MID wrapping a base terrain material (binds volume textures + shadow params).
      *  Returns Base unchanged-wrapped, or nullptr if Base is null. */
     UMaterialInstanceDynamic* GetOrCreateTerrainMID(UMaterialInterface* Base);
@@ -677,9 +680,19 @@ public:
     // Set to true during EndPlay — async tasks check this before accessing UObjects
     std::atomic<bool> bShuttingDown{false};
 
+    // Set during editor-driven generation mutations; distinct from teardown/shutdown semantics.
+    // Active pendant les mutations de génération lancées par l'éditeur, sans signifier la destruction.
+    std::atomic<bool> bGenerationPaused{false};
+
     // Number of async tasks currently running — EndPlay waits for this to reach 0
     std::atomic<int32> ActiveTaskCount{0};
 
+    FORCEINLINE bool ShouldAbortWork() const
+    {
+        return bShuttingDown.load(std::memory_order_relaxed)
+            || bGenerationPaused.load(std::memory_order_relaxed);
+    }
+
     // Player's level-0 tile coord (= chunk coord). The desired set is rebuilt when this changes.
     FIntVector CurrentCenterChunk = FIntVector::ZeroValue;