fix(xcf_stack): run last, configurable timeout, tolerate GIMP exit hang

Defer xcf_stack to the final pipeline step via runs_last, expose per-image
timeout (default 120s), accept XCF output when gimp-console hangs on quit,
and discover .xcf outputs for resume.

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

imagepipeline/modules/base.py

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