feat: optional step_id for custom output folder names

Allow readable step folders and xcf_stack layer labels via p.step(..., step_id=...)
while keeping the per-module auto counter unchanged.

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

tests/test_resume.py

@@ -178,6 +178,34 @@ class TestPipelineResume:
         assert "Reused external output for imagemagick_grayscale_01" in output
         assert (root / "imagemagick_grayscale_01" / "photo_a.png").exists()
 
+    @pytest.mark.skipif(not shutil.which("magick"), reason="ImageMagick not installed")
+    def test_existing_outputs_with_custom_step_id(
+        self, input_dir: Path, output_base: Path, tmp_path: Path, capsys
+    ) -> None:
+        external = tmp_path / "external_gray"
+        external.mkdir()
+        for src in input_dir.iterdir():
+            if src.is_file():
+                make_png(external / src.name, width=4, height=4, rgb=(10, 20, 30))
+
+        with Pipeline(
+            name="custom_id_external",
+            input_dir=input_dir,
+            output_base=output_base,
+            verbose=True,
+            existing_outputs={"input_bokeh": external},
+        ) as p:
+            reused = p.step(
+                "imagemagick_grayscale", inputs="input", step_id="input_bokeh"
+            )
+            p.step("imagemagick_grayscale", inputs=reused)
+            root = p.run()
+
+        output = capsys.readouterr().out
+        assert "Reused external output for input_bokeh" in output
+        assert (root / "input_bokeh" / "photo_a.png").exists()
+        assert not (root / "imagemagick_grayscale_01").exists()
+
 
 class TestMaterializeExternal:
     def test_links_files_by_stem(self, tmp_path: Path) -> None: