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/Private/SAssetExportDialog.h

@@ -0,0 +1,75 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+#include "AssetUsageAuditTypes.h"
+
+/** What "export the ticked files" does. */
+enum class EFileExportMode : uint8
+{
+    /** Byte-exact copies of the package files. Loads nothing. */
+    CopyPackages,
+
+    /** Meshes to FBX, textures to PNG and so on, through the engine's exporters. */
+    ConvertToExchangeFormat
+};
+
+/**
+ * Everything the export dialog collects.
+ *
+ * Lives here rather than in the panel so the two headers do not have to include each other: the
+ * panel needs the mode for its persisted state, and the dialog needs the whole request.
+ */
+struct FAssetExportRequest
+{
+    EFileExportMode Mode = EFileExportMode::CopyPackages;
+
+    /** Absolute destination. Pre-filled from settings, editable in the dialog. */
+    FString TargetDirectory;
+
+    EExportLayout Layout = EExportLayout::Flat;
+
+    /** Only meaningful with FolderPerAsset; the dialog disables it otherwise. */
+    bool bGroupDependenciesByType = false;
+
+    bool bIncludeDependencies = true;
+
+    /**
+     * Write AssetUsageAudit.manifest.json beside the exported files.
+     *
+     * On by default: the Flat and FolderPerAsset layouts destroy the reference graph by design, and
+     * the manifest is the only record of what pointed at what. Meaningless under Migrate, where the
+     * engine keeps the paths intact.
+     */
+    bool bWriteManifest = true;
+
+    EExportCollisionPolicy CollisionPolicy = EExportCollisionPolicy::Index;
+};
+
+/**
+ * Show the modal export dialog.
+ *
+ * Modal on purpose. The alternative - a dockable options panel - lets the ticked set change while
+ * the options are open, so the figures on screen would describe a selection that no longer exists.
+ *
+ * @param InOutRequest  Seeded with the current defaults; overwritten with the user's choices.
+ * @param TickedCount   Shown in the dialog so the user can see what they are about to act on.
+ * @return true if the user pressed Export, false on Cancel or on closing the window.
+ */
+bool ShowAssetExportDialog(FAssetExportRequest& InOutRequest, int32 TickedCount);
+
+/**
+ * Why a folder cannot be used as a Migrate destination, or empty text when it can.
+ *
+ * Mirrors the two checks UAssetToolsImpl performs, because it performs them *after* being called
+ * and reports the failure only to the Output Log - so an export that cannot possibly work would
+ * otherwise look like a button that does nothing.
+ *
+ * The rules, from the engine:
+ *   1. the path must end in /Content/  (MigratePackages_ReportConfirmed);
+ *   2. the folder one level up must hold a .uproject, or exactly one .uplugin
+ *      (FPackageMigrationImpl::GetMountPointRootPath) - that is how the destination's mount point
+ *      is derived, and without it Migrate aborts.
+ */
+FText ValidateMigrateDestination(const FString& Directory);