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/AssetDependencyClosure.h

@@ -0,0 +1,128 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+
+class IAssetRegistry;
+
+/**
+ * Everything a set of packages needs in order to be usable somewhere else.
+ *
+ * Exists because "export the ticked meshes" without this produces meshes that open pink: the
+ * package file of a StaticMesh holds no materials and no textures, only references to them. The
+ * same applies to a Niagara system and its sprites, and to a Blueprint and everything it spawns.
+ *
+ * Deliberately a separate step rather than something FAssetUsageExporter does internally. The
+ * exporter stays a dumb file copier that needs no Asset Registry and is testable without one, and
+ * the caller keeps the chance to show "40 ticked, 380 will be written" before anything is copied -
+ * which matters, because that ratio surprises people.
+ *
+ * This is NOT IAssetTools::MigratePackages. Migrate walks the same closure but also decides where
+ * files go, prompts, and cannot be told to stop at a folder boundary. Here the traversal is bounded
+ * by the same exclusion list the audit uses, and engine content is left out by default because the
+ * destination project already has it.
+ */
+namespace AssetDependencyClosure
+{
+    struct FOptions
+    {
+        /**
+         * Package prefixes not to descend into, in either /Game/... or Content/... form.
+         * Normally the audit's own exclusion list, so an export cannot pull in content the report
+         * deliberately ignores.
+         */
+        TArray<FString> ExcludePackagePaths;
+
+        /**
+         * Include /Engine and /Temp packages in the result.
+         *
+         * Off by default. A mesh using DefaultMaterial genuinely depends on /Engine content, but
+         * copying it into the export folder is almost never what someone wants - the destination
+         * project ships the same file, and overwriting it there is worse than useless.
+         */
+        bool bIncludeEnginePackages = false;
+
+        /**
+         * Stop after this many hops from a seed. Zero means no limit.
+         *
+         * A limit is a blunt instrument and changes the answer rather than just shortening it, so it
+         * is off by default. It exists for the "just the materials, not the whole graph" case.
+         */
+        int32 MaxDepth = 0;
+
+        /** Called as (Visited, Queued). Return false to stop; the partial result is still returned. */
+        TFunction<bool(int32, int32)> OnProgress;
+    };
+
+    // Exported: Summarise is defined out of line, so the Editor module cannot link without this.
+    struct ASSETUSAGEAUDITCORE_API FStats
+    {
+        /** Seeds that were valid package names to begin with. */
+        int32 SeedCount = 0;
+
+        /** Size of the returned array, seeds included. */
+        int32 TotalCount = 0;
+
+        /** Reached but dropped, per reason. Kept apart so a surprising result can be explained. */
+        int32 SkippedScript = 0;
+        int32 SkippedEngine = 0;
+        int32 SkippedExcluded = 0;
+
+        /**
+         * Maps reached through an ordinary asset rather than through a level, and therefore not
+         * followed - nor included.
+         *
+         * The same rule FLevelUsageResolver applies, and for the same measured reason: on WP_Main
+         * the chain BP_FirstPersonGameMode -> PDA_MenuSystemConfig -> L_MainLevel drags in 9994
+         * packages belonging to a different map. Without this the export dialog would quote an
+         * honest number for a wrong set.
+         *
+         * Excluded rather than merely not expanded, unlike in the audit. The exporter expands any
+         * .umap it is handed into its One File Per Actor packages, so including the map file would
+         * pull the foreign level's contents back in through the exporter instead of the closure.
+         */
+        int32 ForeignLevelsSkipped = 0;
+
+        /** Deepest hop count actually reached. Tells you whether MaxDepth did anything. */
+        int32 DeepestHop = 0;
+
+        bool bStoppedEarly = false;
+
+        FString Summarise() const;
+    };
+
+    /**
+     * Seeds plus everything they depend on, transitively.
+     *
+     * Seeds come first and in their original order, so a caller can still tell what was asked for.
+     * The traversal uses AssetUsageAudit::MakeTraversalQuery() - package category, no requirements -
+     * for the same reason the audit does: a Hard-only query silently drops every soft edge, which
+     * includes every One File Per Actor package and every TSoftObjectPtr a Blueprint resolves.
+     */
+    /** One ticked asset and everything it pulls in, the seed first. */
+    struct FSeedClosure
+    {
+        FName Seed;
+        TArray<FName> Packages;
+    };
+
+    /**
+     * Closure per seed, rather than one closure over all of them.
+     *
+     * Needed by the folder-per-asset layout: a texture shared by forty meshes has to appear in all
+     * forty folders, and the flat Gather deliberately returns it once. Costs one traversal per
+     * seed, so it is the slower call by construction - use Gather when a single list will do.
+     */
+    ASSETUSAGEAUDITCORE_API TArray<FSeedClosure> GatherPerSeed(
+        IAssetRegistry& Registry,
+        const TArray<FName>& Seeds,
+        const FOptions& Options,
+        FStats& OutStats);
+
+    ASSETUSAGEAUDITCORE_API TArray<FName> Gather(
+        IAssetRegistry& Registry,
+        const TArray<FName>& Seeds,
+        const FOptions& Options,
+        FStats& OutStats);
+}