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/AssetUsageAuditCore.Build.cs

@@ -0,0 +1,32 @@
+using UnrealBuildTool;
+
+public class AssetUsageAuditCore : ModuleRules
+{
+    public AssetUsageAuditCore(ReadOnlyTargetRules Target) : base(Target)
+    {
+        PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
+
+        // INVARIANT: this module must stay free of UI and of the editor-only asset pipeline.
+        // Do NOT add: UnrealEd, AssetTools, ToolMenus, Slate, SlateCore, EditorSubsystem.
+        // AssetTools is editor-only transitively (it puts UnrealEd in PublicDependencyModuleNames),
+        // so pulling it in here would silently make the whole analysis undrivable from a commandlet.
+        PublicDependencyModuleNames.AddRange(new string[]
+        {
+            "Core",
+            "CoreUObject",
+            "AssetRegistry"
+        });
+
+        PrivateDependencyModuleNames.AddRange(new string[]
+        {
+            // Engine is a Runtime module and is available in commandlets, so it does not
+            // compromise the headless goal. It is needed for ULevel::GetExternalActorsPaths and
+            // ScanLevelAssets - the OFPA folder layout must never be string-built, because
+            // Content Bundles inject /CB/<Guid>/, External Data Layers inject /EDL/<UID>/, and
+            // plugins can register further roots through delegates.
+            "Engine",
+            "Json",
+            "Projects"
+        });
+    }
+}