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"],
                     }
                 }