feat: asset usage audit plugin

Editor tool for the LA and 3D departments: which assets each level uses,
which are used nowhere, and export of a chosen set.

Three modules. AssetUsageAuditCore holds the whole analysis and links no
UI and no editor-only asset pipeline, so it stays runnable from a
commandlet; AssetUsageAuditEditor holds the Slate panel and everything
that needs UnrealEd or AssetTools; AssetUsageAuditTests holds 152 specs.

Load-bearing decisions, each of which produces a wrong answer if undone:

- Dependency queries are always Package + NoRequirements, never Hard. The
  map-to-external-actor edges the OFPA gatherer emits carry Game|Build
  without Hard, so a Hard query drops all 16117 external actor packages
  in this project. There is deliberately no Hard constant in the code.
- Crossing into another map is allowed only from a level or its external
  actor package. Without that rule WP_Main reported 18136 assets, of
  which 9994 belonged to L_MainLevel, reached through the GameMode.
- The verdict has five states, never a bool. The registry cannot see
  FMOD events, DataTable rows or string-built paths; those are Unknown,
  and the tool never proposes a deletion.
- Copying .uasset files does not preserve references - they are stored as
  full package paths. Only the Migrate layout produces something Unreal
  can open; the others write a manifest so the graph can be rebuilt.

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

Source/AssetUsageAuditCore/Private/AssetUsageAuditCoreModule.cpp

@@ -0,0 +1,17 @@
+#include "AssetUsageAuditCoreModule.h"
+
+DEFINE_LOG_CATEGORY(LogAssetUsageAudit);
+
+#define LOCTEXT_NAMESPACE "FAssetUsageAuditCoreModule"
+
+void FAssetUsageAuditCoreModule::StartupModule()
+{
+}
+
+void FAssetUsageAuditCoreModule::ShutdownModule()
+{
+}
+
+#undef LOCTEXT_NAMESPACE
+
+IMPLEMENT_MODULE(FAssetUsageAuditCoreModule, AssetUsageAuditCore)