Initial commit: Item Interaction Ecosystem plugin (UE5.7)

Server-authoritative, data-driven item/inventory/interaction/carry/equipment/
world-slot/placement/crafting framework. 8 modules, compiles against UE 5.7.

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

Source/ItemInventory/Public/ItemTransaction.h

@@ -0,0 +1,101 @@
+// Copyright ExByte Studios. Item Interaction Ecosystem.
+
+#pragma once
+
+#include "CoreMinimal.h"
+#include "GameplayTagContainer.h"
+#include "ItemInstanceData.h"
+#include "ItemTransaction.generated.h"
+
+class UInventoryComponent;
+
+/** Kind of mutation a transaction step performs (section 10). */
+UENUM(BlueprintType)
+enum class EItemTransactionStepType : uint8
+{
+	AddItem,
+	RemoveItem,
+	UpdateItem,
+	MoveItem
+};
+
+/** One atomic step. Carries enough to apply AND to roll back. */
+USTRUCT(BlueprintType)
+struct ITEMINVENTORY_API FItemTransactionStep
+{
+	GENERATED_BODY()
+
+	UPROPERTY()
+	EItemTransactionStepType Type = EItemTransactionStepType::AddItem;
+
+	UPROPERTY()
+	TWeakObjectPtr<UInventoryComponent> Inventory;
+
+	/** Item payload for AddItem / UpdateItem. */
+	UPROPERTY()
+	FItemInstanceData Item;
+
+	UPROPERTY()
+	FGuid InstanceId;
+
+	UPROPERTY()
+	FGuid ToContainer;
+
+	UPROPERTY()
+	int32 ToSlot = INDEX_NONE;
+
+	// --- runtime (rollback bookkeeping) ---
+	UPROPERTY()
+	bool bApplied = false;
+
+	UPROPERTY()
+	FItemInstanceData UndoSnapshot;
+
+	UPROPERTY()
+	FGuid UndoFromContainer;
+
+	UPROPERTY()
+	int32 UndoFromSlot = INDEX_NONE;
+};
+
+/**
+ * Atomic multi-step item operation (section 10). Build steps, then Apply():
+ * if any step fails, every applied step is rolled back so the inventories are
+ * left untouched. The single correct way to do multi-inventory moves.
+ *
+ * Server-only. Operates through UInventoryComponent's authoritative methods.
+ */
+USTRUCT(BlueprintType)
+struct ITEMINVENTORY_API FItemTransaction
+{
+	GENERATED_BODY()
+
+	UPROPERTY()
+	FGuid TransactionId;
+
+	UPROPERTY()
+	FGameplayTag TransactionType;
+
+	UPROPERTY()
+	TWeakObjectPtr<AActor> Instigator;
+
+	UPROPERTY()
+	TArray<FItemTransactionStep> Steps;
+
+	FItemTransaction() { TransactionId = FGuid::NewGuid(); }
+
+	// --- Builders ---
+	FItemTransaction& Add(UInventoryComponent* Inventory, const FItemInstanceData& Item);
+	FItemTransaction& Remove(UInventoryComponent* Inventory, const FGuid& InstanceId);
+	FItemTransaction& Update(UInventoryComponent* Inventory, const FItemInstanceData& Item);
+	FItemTransaction& Move(UInventoryComponent* Inventory, const FGuid& InstanceId, const FGuid& ToContainerId, int32 ToSlotIndex);
+
+	/** Non-mutating feasibility check of every step. */
+	bool Validate() const;
+
+	/** Applies all steps; on any failure rolls back and returns false. */
+	bool Apply();
+
+	/** Undoes every applied step in reverse order. */
+	void Rollback();
+};