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>

README.md

@@ -48,7 +48,7 @@ my_run_20260527143022/
 └── ...
 ```
 
-Step folders are named `{module_name}_{nn}` (two-digit counter per module name).
+Step folders are named `{module_name}_{nn}` by default (two-digit counter per module name). Pass optional `step_id="input_bokeh"` to `p.step()` for a custom folder name and step reference (see [docs/MODULE_DEVELOPMENT.md](docs/MODULE_DEVELOPMENT.md#step-folder-naming)).
 
 ## Writing Pipelines