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/AssetUsageAuditTests/AssetUsageAuditTests.Build.cs

@@ -19,6 +19,11 @@ public class AssetUsageAuditTests : ModuleRules
         {
             "AssetRegistry",
             "AssetUsageAuditCore",
+            // Only the Migrate live spec needs these, and only ever under EditorContext. The Core
+            // module still must not link them - that invariant is about the analysis being runnable
+            // headless, and a test module is not part of that path.
+            "AssetTools",
+            "UnrealEd",
             "Json",
             "Projects"
         });