fix: xcf_stack GIMP 3 headless script execution

Use gimp-console when available, GIMP 3 Script-Fu API, and temp .scm
batch files so layer stacking works without a display.

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

tests/test_xcf_stack.py

@@ -12,26 +12,10 @@ from imagepipeline.modules.xcf_stack import XcfStackModule
 from imagepipeline.utils.files import find_image_by_stem
 from tests.conftest import make_png
 
-has_gimp = bool(shutil.which("gimp"))
+has_gimp = bool(shutil.which("gimp-console") or shutil.which("gimp"))
 has_magick = bool(shutil.which("magick") or shutil.which("convert"))
 
 
-def _gimp_headless_works() -> bool:
-    gimp = shutil.which("gimp")
-    if not gimp:
-        return False
-    try:
-        from imagepipeline.utils.subprocess import run_command
-
-        run_command([gimp, "-idf", "-b", "(gimp-quit 0)"], timeout=10)
-        return True
-    except RuntimeError:
-        return False
-
-
-has_working_gimp = _gimp_headless_works()
-
-
 class TestFindImageByStem:
     def test_finds_matching_stem_case_insensitively(self, tmp_path: Path) -> None:
         make_png(tmp_path / "Photo.PNG")
@@ -174,7 +158,7 @@ class TestXcfStackRun:
         mock_stack.assert_not_called()
 
 
-@pytest.mark.skipif(not has_working_gimp, reason="GIMP headless not available")
+@pytest.mark.skipif(not has_gimp, reason="GIMP not installed")
 @pytest.mark.skipif(not has_magick, reason="ImageMagick not installed")
 class TestStackImagesToXcfIntegration:
     def test_stacks_pngs_into_xcf(self, tmp_path: Path) -> None:
@@ -198,3 +182,6 @@ class TestStackImagesToXcfIntegration:
 
         assert outfile.is_file()
         assert outfile.stat().st_size > 0
+        names = outfile.read_bytes()
+        assert b"bottom" in names
+        assert b"top" in names