refactor(xcf_stack): use explicit inputs for layer list and scheduling

Replace prior_steps and runs_last with inputs=[...] step refs so GIMP
export waits only on listed layers. Add rezepttest pipeline and
bokeh-oktagon recipe.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_xcf_stack.py

@@ -41,7 +41,7 @@ class TestModuleRegistration:
     def test_get_module_returns_xcf_stack_class(self) -> None:
         assert get_module("xcf_stack") is XcfStackModule
 
-    def test_xcf_stack_runs_last(self) -> None:
+    def test_xcf_stack_waits_for_layer_inputs(self) -> None:
         from imagepipeline.core.runner import PipelineRunner
         from imagepipeline.core.step import StepDefinition
         from imagepipeline.modules.imagemagick_grayscale import ImageMagickGrayscale
@@ -51,7 +51,7 @@ class TestModuleRegistration:
                 step_id="xcf_stack_01",
                 module_name="xcf_stack",
                 module=XcfStackModule,
-                input_refs=["input"],
+                input_refs=["input", "imagemagick_grayscale_01"],
                 params={},
                 output_dir_name="xcf_stack_01",
             ),
@@ -71,7 +71,10 @@ class TestModuleRegistration:
             steps=steps,
         )
         ordered = runner._topological_sort()
-        assert ordered[-1].module_name == "xcf_stack"
+        assert [step.step_id for step in ordered] == [
+            "imagemagick_grayscale_01",
+            "xcf_stack_01",
+        ]
 
 
 class TestExpectedOutputFilenames:
@@ -116,7 +119,7 @@ def _make_stack_fixture(tmp_path: Path) -> dict[str, Path]:
 
 class TestXcfStackRun:
     @patch("imagepipeline.modules.xcf_stack.stack_images_to_xcf")
-    def test_collects_layers_from_input_and_prior_steps(
+    def test_collects_layers_from_explicit_inputs(
         self, mock_stack: object, tmp_path: Path
     ) -> None:
         paths = _make_stack_fixture(tmp_path)
@@ -130,7 +133,8 @@ class TestXcfStackRun:
             params=XcfStackModule.validate_module_params({}),
             pipeline_output_root=paths["root"],
             step_id="xcf_stack_01",
-            prior_steps=[
+            input_layer_dirs=[
+                ("input", paths["input_dir"]),
                 ("step_a", paths["step_a"]),
                 ("step_b", paths["step_b"]),
             ],
@@ -161,7 +165,8 @@ class TestXcfStackRun:
             params=XcfStackModule.validate_module_params({"skip_missing": True}),
             pipeline_output_root=paths["root"],
             step_id="xcf_stack_01",
-            prior_steps=[
+            input_layer_dirs=[
+                ("input", paths["input_dir"]),
                 ("step_a", paths["step_a"]),
                 ("step_b", paths["step_b"]),
             ],
@@ -187,7 +192,8 @@ class TestXcfStackRun:
             params=XcfStackModule.validate_module_params({"skip_missing": False}),
             pipeline_output_root=paths["root"],
             step_id="xcf_stack_01",
-            prior_steps=[
+            input_layer_dirs=[
+                ("input", paths["input_dir"]),
                 ("step_a", paths["step_a"]),
                 ("step_b", paths["step_b"]),
             ],