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>

imagepipeline/modules/base.py

@@ -14,7 +14,6 @@ class BaseModule(ABC):
 
     name: ClassVar[str]
     description: ClassVar[str] = ""
-    runs_last: ClassVar[bool] = False
     supported_input_formats: ClassVar[tuple[str, ...]] = (
         ".jpg",
         ".jpeg",