feat: add SDXL Turbo image generator wrapper

tests/test_asset_generator.py

@@ -0,0 +1,52 @@
+from unittest.mock import MagicMock, patch
+from PIL import Image
+
+from server.asset_generator import AssetGenerator
+
+
+class TestAssetGenerator:
+    @patch("server.asset_generator.AutoPipelineForText2Image")
+    def test_init_loads_model(self, mock_pipeline_cls):
+        """Generator loads the SDXL Turbo pipeline on init."""
+        mock_pipe = MagicMock()
+        mock_pipeline_cls.from_pretrained.return_value = mock_pipe
+        mock_pipe.to.return_value = mock_pipe
+
+        gen = AssetGenerator(device="cpu")
+        mock_pipeline_cls.from_pretrained.assert_called_once()
+
+    @patch("server.asset_generator.AutoPipelineForText2Image")
+    def test_generate_returns_bytes(self, mock_pipeline_cls):
+        """Generate returns PNG bytes."""
+        mock_pipe = MagicMock()
+        mock_pipeline_cls.from_pretrained.return_value = mock_pipe
+        mock_pipe.to.return_value = mock_pipe
+
+        # Mock pipeline output
+        fake_image = Image.new("RGB", (512, 512), color="black")
+        mock_result = MagicMock()
+        mock_result.images = [fake_image]
+        mock_pipe.return_value = mock_result
+
+        gen = AssetGenerator(device="cpu")
+        data = gen.generate("dark void, horror")
+        assert isinstance(data, bytes)
+        assert len(data) > 0
+
+    @patch("server.asset_generator.AutoPipelineForText2Image")
+    def test_generate_uses_negative_prompt(self, mock_pipeline_cls):
+        """Generate passes the negative prompt to the pipeline."""
+        mock_pipe = MagicMock()
+        mock_pipeline_cls.from_pretrained.return_value = mock_pipe
+        mock_pipe.to.return_value = mock_pipe
+
+        fake_image = Image.new("RGB", (512, 512), color="black")
+        mock_result = MagicMock()
+        mock_result.images = [fake_image]
+        mock_pipe.return_value = mock_result
+
+        gen = AssetGenerator(device="cpu")
+        gen.generate("test prompt")
+
+        call_kwargs = mock_pipe.call_args
+        assert "negative_prompt" in call_kwargs.kwargs