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/AssetUsageAuditEditor/Private/SAssetImportDialog.h

@@ -0,0 +1,16 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+
+/**
+ * Show the import window: pick a folder, review what it holds, tick what to bring in.
+ *
+ * Modal, like the export window and for the same reason - the list describes a folder on disk at a
+ * moment in time, and letting the project change underneath it would make the "already exists" and
+ * "missing dependency" columns lie.
+ *
+ * Blocks until the window closes. Nothing is written unless Import is pressed.
+ */
+void ShowAssetImportDialog();