test(domain): add wave197 asset/asset_library compat tests (+12)

Add tests for compatibility layer modules:
- asset.py: AssetType constants + re-exports
- asset_library.py: LibraryKind constants + re-exports

This completes domain layer zero-test module cleanup.

tests/unit/domain/test_asset_compat.py

@@ -0,0 +1,61 @@
+"""asset / asset_library 兼容层单元测试."""
+
+from __future__ import annotations
+
+from domain.asset import Asset, AssetStatus, AssetType, ClassificationStatus
+from domain.asset_library import AssetLibrary, AssetLibraryKind, LibraryKind
+
+
+class TestAssetType:
+    """AssetType 常量类测试."""
+
+    def test_video_value(self):
+        assert AssetType.VIDEO == "video"
+
+    def test_image_value(self):
+        assert AssetType.IMAGE == "image"
+
+    def test_audio_value(self):
+        assert AssetType.AUDIO == "audio"
+
+    def test_three_types(self):
+        assert AssetType.VIDEO
+        assert AssetType.IMAGE
+        assert AssetType.AUDIO
+
+
+class TestAssetReexports:
+    """asset.py 重导出测试."""
+
+    def test_asset_reexported(self):
+        # Asset 类从 entities 转发，确认可访问
+        assert Asset is not None
+
+    def test_asset_status_reexported(self):
+        assert AssetStatus is not None
+
+    def test_classification_status_reexported(self):
+        assert ClassificationStatus is not None
+
+
+class TestLibraryKind:
+    """LibraryKind 常量类测试."""
+
+    def test_video_value(self):
+        assert LibraryKind.VIDEO == AssetLibraryKind.VIDEO
+
+    def test_voice_value(self):
+        assert LibraryKind.VOICE == AssetLibraryKind.VOICE
+
+    def test_image_value(self):
+        assert LibraryKind.IMAGE == AssetLibraryKind.IMAGE
+
+
+class TestAssetLibraryReexports:
+    """asset_library.py 重导出测试."""
+
+    def test_asset_library_reexported(self):
+        assert AssetLibrary is not None
+
+    def test_asset_library_kind_reexported(self):
+        assert AssetLibraryKind is not None