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/Public/IndirectReferenceScanner.h

@@ -0,0 +1,66 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+
+/**
+ * One /Game path literal found in a text file, with enough context to check it by hand.
+ */
+struct FIndirectReference
+{
+    /** Package name the literal resolved to, e.g. /Game/Space/Core/GameModes/BP_FirstPersonGameMode. */
+    FName PackageName;
+
+    /** Absolute path of the file the literal was found in. */
+    FString SourceFile;
+
+    /** 1-based line number. */
+    int32 LineNumber = 0;
+
+    /** True when the hit came from Config/, false when from Source/. */
+    bool bFromConfig = false;
+
+    FString ToProvenanceString() const;
+};
+
+/**
+ * Finds asset paths referenced from text rather than from a package.
+ *
+ * This exists because of a concrete, measured failure mode on this project: Config/DefaultEngine.ini
+ * names BP_FirstPersonGameMode, BP_MenuSystemGameInstance, RefinedMenuMap and Gyms_Geoda as plain
+ * strings. No asset references them, so the Asset Registry reports zero referencers and a naive
+ * audit calls the project's GameMode unused.
+ *
+ * It is a mitigation, not a solution. It cannot see a path assembled at runtime by concatenation,
+ * an OpenLevel(FName) call, or an FMOD event path - those stay Unknown, deliberately.
+ */
+class ASSETUSAGEAUDITCORE_API FIndirectReferenceScanner
+{
+public:
+    struct FOptions
+    {
+        /** Absolute directories to scan. Defaults to <Project>/Config and <Project>/Source. */
+        TArray<FString> Directories;
+
+        /** File extensions to read, lowercase, with the dot. */
+        TArray<FString> Extensions = { TEXT(".ini"), TEXT(".cpp"), TEXT(".h"), TEXT(".cs") };
+
+        /** Skip files larger than this; a multi-megabyte generated file is never a reference site. */
+        int64 MaxFileSizeBytes = 8 * 1024 * 1024;
+    };
+
+    /** Scan and return every distinct package path found, with provenance. */
+    static TArray<FIndirectReference> Scan(const FOptions& Options);
+
+    /** Default options: <Project>/Config and <Project>/Source. */
+    static FOptions MakeDefaultOptions();
+
+    /**
+     * Extract /Game path literals from one line of text.
+     *
+     * Exposed for testing: the trailing-_C strip and the delimiter set are where this goes wrong.
+     * "/Game/X/BP_Y.BP_Y_C" and "/Game/X/BP_Y" must both yield the package /Game/X/BP_Y.
+     */
+    static void ExtractGamePathsFromLine(FStringView Line, TArray<FString>& OutPackageNames);
+};