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/AssetUsageReportWriter.h

@@ -0,0 +1,68 @@
+// NextGenium 2026. Asset Usage Audit.
+
+#pragma once
+
+#include "CoreMinimal.h"
+
+struct FAssetUsageAuditResult;
+
+/**
+ * Writes the audit result as JSON and CSV.
+ *
+ * Both come from one in-memory result in a single call, so the two files can never disagree
+ * about the same run. JSON is the machine-readable form (nested level lists, filter metadata);
+ * CSV is the flat form a human opens in Excel.
+ */
+class ASSETUSAGEAUDITCORE_API FAssetUsageReportWriter
+{
+public:
+    struct FOptions
+    {
+        /** Absolute directory to write into. Created if missing. */
+        FString OutputDirectory;
+
+        /** Base filename without extension; ".json" and ".csv" are appended. */
+        FString BaseFileName = TEXT("AssetUsageReport");
+
+        bool bWriteJson = true;
+        bool bWriteCsv = true;
+
+        /**
+         * Write a UTF-8 BOM at the start of the CSV.
+         *
+         * Excel misreads UTF-8 without it and mangles every non-ASCII asset name, which on this
+         * project means the Cyrillic folder and asset names come out as garbage.
+         */
+        bool bCsvUtf8Bom = true;
+
+        /** CSV field separator. Semicolon suits locales where the comma is a decimal separator. */
+        TCHAR CsvDelimiter = TEXT(';');
+
+        /** Separator for multi-valued fields inside one CSV cell, e.g. the level list. */
+        FString CsvMultiValueSeparator = TEXT("|");
+    };
+
+    struct FResult
+    {
+        bool bSuccess = false;
+        FString JsonPath;
+        FString CsvPath;
+        FString ErrorMessage;
+    };
+
+    static FResult Write(const FAssetUsageAuditResult& AuditResult, const FOptions& Options);
+
+    /** Serialize to a JSON string without touching disk. Exposed for tests. */
+    static FString BuildJson(const FAssetUsageAuditResult& AuditResult);
+
+    /** Serialize to a CSV string without touching disk. Exposed for tests. */
+    static FString BuildCsv(const FAssetUsageAuditResult& AuditResult, const FOptions& Options);
+
+    /**
+     * Quote and escape one CSV field per RFC 4180.
+     *
+     * Exposed because this is where CSV writers usually break: a field containing the delimiter,
+     * a quote or a newline must be quoted, and embedded quotes doubled.
+     */
+    static FString EscapeCsvField(const FString& Field, TCHAR Delimiter);
+};