feat: support multiple reference images with model-aware API mapping

Replace singular reference_image field with reference_images list to
support an arbitrary number of reference images. Map them to the correct
BFL API parameter names based on model family:
- flux-2-*: input_image, input_image_2, ..., input_image_8
- flux-kontext-*: input_image, input_image_2, ..., input_image_4
- flux 1.x: image_prompt (single)

BREAKING CHANGE: reference_image config field renamed to reference_images (list).

tests/test_builder.py

@@ -91,7 +91,7 @@ class TestCollectHelpers:
                     "out.png": {
                         "prompt": "x",
                         "inputs": ["input.txt"],
-                        "reference_image": "ref.png",
+                        "reference_images": ["ref.png"],
                         "control_images": [],
                     }
                 }
@@ -110,7 +110,7 @@ class TestCollectHelpers:
                         "prompt": "x",
                         "width": 512,
                         "height": 256,
-                        "reference_image": "ref.png",
+                        "reference_images": ["ref.png"],
                     }
                 }
             }
@@ -118,7 +118,7 @@ class TestCollectHelpers:
         params = _collect_extra_params("out.png", config)
         assert params["width"] == 512
         assert params["height"] == 256
-        assert params["reference_image"] == "ref.png"
+        assert params["reference_images"] == ("ref.png",)
 
     def test_collect_extra_params_empty(self, write_config: WriteConfig) -> None:
         config = write_config({"targets": {"out.txt": {"prompt": "x"}}})
@@ -131,7 +131,7 @@ class TestCollectHelpers:
                     "out.png": {
                         "prompt": "x",
                         "inputs": ["a.txt"],
-                        "reference_image": "ref.png",
+                        "reference_images": ["ref.png"],
                         "control_images": ["c1.png", "c2.png"],
                     }
                 }

tests/test_config.py

@@ -45,7 +45,7 @@ class TestLoadConfig:
                     "width": 1920,
                     "height": 480,
                     "inputs": ["ref.png"],
-                    "reference_image": "ref.png",
+                    "reference_images": ["ref.png"],
                     "control_images": ["ctrl.png"],
                 },
                 "story.md": {
@@ -66,7 +66,7 @@ class TestLoadConfig:
         assert banner.model == "flux-dev"
         assert banner.width == 1920
         assert banner.height == 480
-        assert banner.reference_image == "ref.png"
+        assert banner.reference_images == ["ref.png"]
         assert banner.control_images == ["ctrl.png"]
 
         story = config.targets["story.md"]

tests/test_graph.py

@@ -66,7 +66,7 @@ class TestBuildGraph:
     ) -> None:
         _ = (project_dir / "ref.png").write_bytes(b"\x89PNG")
         config = write_config(
-            {"targets": {"out.png": {"prompt": "x", "reference_image": "ref.png"}}}
+            {"targets": {"out.png": {"prompt": "x", "reference_images": ["ref.png"]}}}
         )
         graph = build_graph(config, project_dir)
         assert graph.has_edge("ref.png", "out.png")

tests/test_providers.py

@@ -120,13 +120,13 @@ class TestImageProvider:
             assert inputs["width"] == 1920
             assert inputs["height"] == 480
 
-    async def test_image_with_reference_image(
+    async def test_image_with_reference_image_flux1(
         self, project_dir: Path, image_bytes: bytes
     ) -> None:
         ref_path = project_dir / "ref.png"
         _ = ref_path.write_bytes(b"reference image data")
 
-        target_config = TargetConfig(prompt="Like this", reference_image="ref.png")
+        target_config = TargetConfig(prompt="Like this", reference_images=["ref.png"])
         bfl_result, mock_http = _make_bfl_mocks(image_bytes)
 
         with (
@@ -142,7 +142,7 @@ class TestImageProvider:
                 target_name="out.png",
                 target_config=target_config,
                 resolved_prompt="Like this",
-                resolved_model="flux-kontext-pro",
+                resolved_model="flux-pro-1.1",
                 project_dir=project_dir,
             )
 
@@ -151,6 +151,71 @@ class TestImageProvider:
             assert "image_prompt" in inputs
             assert inputs["image_prompt"] == encode_image_b64(ref_path)
 
+    async def test_image_with_reference_images_flux2(
+        self, project_dir: Path, image_bytes: bytes
+    ) -> None:
+        ref1 = project_dir / "ref1.png"
+        ref2 = project_dir / "ref2.png"
+        _ = ref1.write_bytes(b"ref1 data")
+        _ = ref2.write_bytes(b"ref2 data")
+
+        target_config = TargetConfig(
+            prompt="Combine", reference_images=["ref1.png", "ref2.png"]
+        )
+        bfl_result, mock_http = _make_bfl_mocks(image_bytes)
+
+        with (
+            patch("bulkgen.providers.image.BFLClient") as mock_cls,
+            patch("bulkgen.providers.image.httpx.AsyncClient") as mock_http_cls,
+        ):
+            mock_generate = AsyncMock(return_value=bfl_result)
+            mock_cls.return_value.generate = mock_generate
+            mock_http_cls.return_value = mock_http
+
+            provider = ImageProvider(api_key="test-key")
+            await provider.generate(
+                target_name="out.png",
+                target_config=target_config,
+                resolved_prompt="Combine",
+                resolved_model="flux-2-pro",
+                project_dir=project_dir,
+            )
+
+            call_args = mock_generate.call_args
+            inputs = call_args[0][1]
+            assert inputs["input_image"] == encode_image_b64(ref1)
+            assert inputs["input_image_2"] == encode_image_b64(ref2)
+
+    async def test_image_with_reference_image_kontext(
+        self, project_dir: Path, image_bytes: bytes
+    ) -> None:
+        ref_path = project_dir / "ref.png"
+        _ = ref_path.write_bytes(b"reference image data")
+
+        target_config = TargetConfig(prompt="Edit", reference_images=["ref.png"])
+        bfl_result, mock_http = _make_bfl_mocks(image_bytes)
+
+        with (
+            patch("bulkgen.providers.image.BFLClient") as mock_cls,
+            patch("bulkgen.providers.image.httpx.AsyncClient") as mock_http_cls,
+        ):
+            mock_generate = AsyncMock(return_value=bfl_result)
+            mock_cls.return_value.generate = mock_generate
+            mock_http_cls.return_value = mock_http
+
+            provider = ImageProvider(api_key="test-key")
+            await provider.generate(
+                target_name="out.png",
+                target_config=target_config,
+                resolved_prompt="Edit",
+                resolved_model="flux-kontext-pro",
+                project_dir=project_dir,
+            )
+
+            call_args = mock_generate.call_args
+            inputs = call_args[0][1]
+            assert inputs["input_image"] == encode_image_b64(ref_path)
+
     async def test_image_no_sample_url_raises(self, project_dir: Path) -> None:
         target_config = TargetConfig(prompt="x")