Mesh Optimizer: sibling StaticMesh duplicate remover (UE 5.7)

Editor plugin that detects geometrically-identical sibling StaticMeshes across a
level, rebases each placement onto one canonical mesh with a corrected transform
(W' = D * W, verified by exact vertex matching), and can collapse groups into HISM.
Native Slate tool panel + BlueprintCallable UOptimizerSubsystem.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Source/OptimizerEditor/Public/OptimizerSubsystem.h

@@ -0,0 +1,76 @@
+// Copyright IHY.
+#pragma once
+
+#include "CoreMinimal.h"
+#include "EditorSubsystem.h"
+#include "OptimizerTypes.h"
+#include "OptimizerSubsystem.generated.h"
+
+class UStaticMesh;
+class UStaticMeshComponent;
+
+/**
+ * Editor subsystem driving the Mesh Optimizer pipeline. The Editor Utility Widget calls these:
+ *   1. ScanLevel  -> dry-run plan (sibling groups + per-placement corrected transforms)
+ *   2. ApplyUnify -> reassign every sibling placement to its canonical mesh with the fixed transform
+ *   3. BuildHISM  -> collapse each group into one HierarchicalInstancedStaticMesh
+ * Steps 2 and 3 each run inside a single undoable transaction. BuildHISM is independent of ApplyUnify
+ * (it uses the canonical mesh + planned transforms directly).
+ */
+UCLASS()
+class OPTIMIZEREDITOR_API UOptimizerSubsystem : public UEditorSubsystem
+{
+	GENERATED_BODY()
+
+public:
+	/** Dry run: scan the level, group sibling meshes, plan corrected transforms. Mutates nothing. */
+	UFUNCTION(BlueprintCallable, Category = "Optimizer")
+	FOptimizerScanResult ScanLevel(const FOptimizerScanSettings& Settings);
+
+	/** Reassign all non-instanced sibling placements to their canonical mesh + corrected transform. Returns count changed. */
+	UFUNCTION(BlueprintCallable, Category = "Optimizer")
+	int32 ApplyUnify();
+
+	/** Build one HISM actor per group from the planned transforms. Returns number of HISM actors created. */
+	UFUNCTION(BlueprintCallable, Category = "Optimizer")
+	int32 BuildHISM(bool bDestroyOriginals = true);
+
+	/** Discard the current plan. */
+	UFUNCTION(BlueprintCallable, Category = "Optimizer")
+	void ClearPlan();
+
+	UFUNCTION(BlueprintCallable, Category = "Optimizer")
+	bool HasPlan() const;
+
+private:
+	/** One placement of a mesh in the level (a component, or one instance inside an ISM/HISM). */
+	struct FPlacement
+	{
+		TWeakObjectPtr<UStaticMeshComponent> Component;
+		int32 InstanceIndex = INDEX_NONE;   // >= 0 when this is an instance inside an ISM/HISM
+		TWeakObjectPtr<UStaticMesh> Mesh;
+		FTransform World = FTransform::Identity;
+		FTransform PlannedWorld = FTransform::Identity;
+		bool bNeedsTransformFix = false;
+		bool bShearRejected = false;
+		bool bInstanced = false;            // inside an ISM/HISM -> ApplyUnify can't swap it (HISM-only)
+	};
+
+	/** A sibling group: distinct meshes that share geometry, collapsed onto one canonical. */
+	struct FGroup
+	{
+		TArray<TWeakObjectPtr<UStaticMesh>> Members;
+		TWeakObjectPtr<UStaticMesh> Canonical;
+		TArray<FPlacement> Placements;
+		float MaxDeviation = 0.0f;
+		bool bHasMirrored = false;
+		bool bHasScaled = false;
+		bool bMaterialMismatch = false;
+		int32 ShearRejected = 0;
+	};
+
+	TArray<FGroup> Groups;
+	FOptimizerScanSettings LastSettings;
+
+	FOptimizerScanResult BuildResultView() const;
+};