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/Private/VoxelContentManager.cpp

@@ -68,17 +68,23 @@ void UVoxelContentManager::NotifyShutdown()
 
     // Wait for in-flight march tasks to finish (they check the flag and bail). Timeout to avoid hangs.
     const double Deadline = FPlatformTime::Seconds() + 3.0;
-    while (GActiveDecoTasks.load(std::memory_order_relaxed) > 0)
-    {
-        if (FPlatformTime::Seconds() > Deadline) break;
-        FPlatformProcess::Yield();
-    }
+    WaitForDecorationTasks(Deadline);
 
     DrainDecoResults();
     ResetGridBuildState(NearGrid);
     ResetGridBuildState(FarGrid);
 }
 
+bool UVoxelContentManager::WaitForDecorationTasks(double Deadline)
+{
+    while (GActiveDecoTasks.load(std::memory_order_relaxed) > 0)
+    {
+        if (FPlatformTime::Seconds() > Deadline) return false;
+        FPlatformProcess::Yield();
+    }
+    return true;
+}
+
 void UVoxelContentManager::DrainDecoResults()
 {
     FDecoCellResult Discard;