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/AssetExportNaming.h

@@ -0,0 +1,57 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+#include "AssetUsageAuditTypes.h"
+
+/**
+ * Collision-handling for export filenames.
+ *
+ * Deliberately a free function over strings with an injectable existence predicate: the rule
+ * ("increment an existing index, do not append a second one") is fiddly on real asset names like
+ * SM_Rock_02_v3, and it must be unit-testable without touching a filesystem or an editor.
+ */
+namespace AssetExportNaming
+{
+    /** Predicate answering "does this filename already exist in the target folder". */
+    using FExistsPredicate = TFunctionRef<bool(const FString& /*FileName*/)>;
+
+    /**
+     * Split a base name into stem and trailing numeric index.
+     *
+     * "Foo"          -> {"Foo",         INDEX_NONE, 0}
+     * "Foo_1"        -> {"Foo",         1,          1}
+     * "Foo_007"      -> {"Foo",         7,          3}   <- padding width preserved
+     * "SM_Rock_02_v3"-> {"SM_Rock_02_v3", INDEX_NONE, 0}  <- v3 is not a numeric suffix
+     * "SM_Rock_02"   -> {"SM_Rock",     2,          2}
+     * "Foo_"         -> {"Foo_",        INDEX_NONE, 0}   <- empty suffix is not an index
+     *
+     * @param BaseName      Filename without extension.
+     * @param OutStem       Portion before the trailing _N, or the whole name when there is none.
+     * @param OutIndex      Parsed index, or INDEX_NONE.
+     * @param OutPadWidth   Digit count of the parsed index, so Foo_007 -> Foo_008 not Foo_8.
+     */
+    ASSETUSAGEAUDITCORE_API void SplitTrailingIndex(const FString& BaseName, FString& OutStem, int32& OutIndex, int32& OutPadWidth);
+
+    /**
+     * Compose a filename from stem, index and padding.
+     * (Foo, INDEX_NONE, 0) -> "Foo"
+     * (Foo, 1, 1)          -> "Foo_1"
+     * (Foo, 8, 3)          -> "Foo_008"
+     */
+    ASSETUSAGEAUDITCORE_API FString ComposeIndexedName(const FString& Stem, int32 Index, int32 PadWidth);
+
+    /**
+     * Resolve a target filename under the given collision policy.
+     *
+     * Overwrite -> returns DesiredFileName unchanged.
+     * Index     -> returns the first non-colliding name, incrementing any existing trailing index.
+     *
+     * @param DesiredFileName  Filename with extension, e.g. "SM_Rock.uasset".
+     * @param Policy           Overwrite or Index.
+     * @param Exists           Predicate over filenames (with extension) in the target folder.
+     * @return Filename with extension that does not collide, or DesiredFileName under Overwrite.
+     */
+    ASSETUSAGEAUDITCORE_API FString ResolveCollision(const FString& DesiredFileName, EExportCollisionPolicy Policy, FExistsPredicate Exists);
+}