feat: import assets from an external folder

Reverse direction of the export: read a folder of .uasset files, recover
each one's original package path, and bring the chosen ones back.

Recovering the path is the hard part - a reference inside a .uasset names
its target by full package path, so a file only works when restored to
where it came from. Four sources are tried and the one used is shown per
row: our manifest, the package header, the folder structure, or nothing.

The package header carries the name on this project's assets, measured,
so a flat export without a manifest still restores correctly. The engine
notes the field is not always written, hence the chain rather than one
check.

Import refuses to destroy work: existing assets are skipped unless
overwrite is explicitly on, and a package loaded in the editor is never
replaced.

Also verifies Migrate on real data for the first time - asset and its
dependency land at the correct package paths.

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

Source/AssetUsageAuditCore/Public/AssetImporter.h

@@ -0,0 +1,87 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+#include "AssetImportScanner.h"
+
+class IAssetRegistry;
+
+/**
+ * Copies scanned files into the project at their recovered package paths.
+ *
+ * A byte copy is enough: a .uasset is self-contained and its references are resolved by package
+ * path, so putting the file where the path says produces a working asset without loading anything.
+ * That is also why the scanner's job - deciding the path - is the hard half and this is the easy one.
+ *
+ * What this refuses to do matters more than what it does. Importing is the one operation here that
+ * can destroy work: writing over /Game/Art/SM_Rock replaces whatever the project had under that
+ * name, with no undo and no trace. So an existing target is skipped unless overwriting was asked
+ * for explicitly, and a package already loaded in the editor is never written over at all.
+ */
+namespace AssetImporter
+{
+    struct FOptions
+    {
+        /**
+         * Replace assets that already exist at the target path.
+         *
+         * Off by default and deliberately awkward to turn on: the scanner reports the count up
+         * front so the choice is made knowingly rather than discovered afterwards.
+         */
+        bool bOverwriteExisting = false;
+
+        /**
+         * Where to put files whose package path could not be recovered, e.g. "/Game/Imported".
+         *
+         * Empty means skip them. They can be imported, but their references will not resolve, so
+         * putting them somewhere by default would quietly fill the project with broken assets.
+         */
+        FString UnresolvedDestinationPath;
+
+        /** Called as (Done, Total). Return false to stop; files already written are kept. */
+        TFunction<bool(int32, int32)> OnProgress;
+    };
+
+    struct ASSETUSAGEAUDITCORE_API FResult
+    {
+        bool bSuccess = false;
+
+        int32 FilesImported = 0;
+
+        /** Target existed and overwriting was not requested. */
+        int32 SkippedExisting = 0;
+
+        /** No recoverable package path and no fallback folder was given. */
+        int32 SkippedUnresolved = 0;
+
+        /**
+         * Target package is loaded in this editor session.
+         *
+         * Counted separately because it is not a user choice to make: replacing the file under a
+         * loaded package leaves the editor holding stale objects that will be saved back over the
+         * new file. The asset has to be closed first.
+         */
+        int32 SkippedLoaded = 0;
+
+        bool bCancelled = false;
+
+        TArray<FString> Errors;
+
+        /** Absolute paths written, for the Asset Registry rescan that makes them visible. */
+        TArray<FString> ImportedFiles;
+
+        FString Summarise() const;
+    };
+
+    /**
+     * Import the given candidates.
+     *
+     * Only candidates the caller wants should be passed in - this applies no filtering of its own
+     * beyond the safety rules above.
+     */
+    ASSETUSAGEAUDITCORE_API FResult Import(
+        IAssetRegistry& Registry,
+        const TArray<AssetImportScanner::FCandidate>& Candidates,
+        const FOptions& Options);
+}