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/AssetUsageAuditEditor/AssetUsageAuditEditor.Build.cs

@@ -0,0 +1,38 @@
+// NextGenium 2026. Asset Usage Audit.
+
+using UnrealBuildTool;
+
+public class AssetUsageAuditEditor : ModuleRules
+{
+    public AssetUsageAuditEditor(ReadOnlyTargetRules Target) : base(Target)
+    {
+        PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
+
+        PublicDependencyModuleNames.AddRange(new string[]
+        {
+            "Core",
+            "AssetUsageAuditCore"
+        });
+
+        // Everything editor-only and UI-shaped belongs here and nowhere else. Keeping this list
+        // out of AssetUsageAuditCore is what lets the same analysis run from a commandlet later.
+        PrivateDependencyModuleNames.AddRange(new string[]
+        {
+            "CoreUObject",
+            "Engine",
+            "AssetRegistry",
+            "AssetTools",               // MigratePackages - the only path that keeps references working
+            "DeveloperSettings",
+            "DesktopPlatform",          // native folder picker for the export destination
+            "EditorFramework",
+            "InputCore",
+            "Projects",
+            "Slate",
+            "SlateCore",
+            "ToolMenus",
+            "ToolWidgets",              // SSearchBox, SSegmentedControl
+            "UnrealEd",
+            "WorkspaceMenuStructure"    // places the tab under Tools in the Window menu
+        });
+    }
+}