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>

AssetUsageAudit.uplugin

@@ -0,0 +1,37 @@
+{
+	"FileVersion": 3,
+	"Version": 1,
+	"VersionName": "0.1",
+	"FriendlyName": "Asset Usage Audit",
+	"Description": "Reports which assets are used on which levels, resolving usage transitively through Blueprints and other assets, and handling hard/soft references and One File Per Actor. Exports selected assets and usage reports.",
+	"Category": "Editor",
+	"CreatedBy": "Next-Genium",
+	"CanContainContent": false,
+	"EnabledByDefault": true,
+	"Modules": [
+		{
+			"Name": "AssetUsageAuditCore",
+			"Type": "UncookedOnly",
+			"LoadingPhase": "Default",
+			"PlatformAllowList": [
+				"Win64"
+			]
+		},
+		{
+			"Name": "AssetUsageAuditEditor",
+			"Type": "Editor",
+			"LoadingPhase": "PostEngineInit",
+			"PlatformAllowList": [
+				"Win64"
+			]
+		},
+		{
+			"Name": "AssetUsageAuditTests",
+			"Type": "UncookedOnly",
+			"LoadingPhase": "Default",
+			"PlatformAllowList": [
+				"Win64"
+			]
+		}
+	]
+}