Introduce the `layer-separation` generator (edit-elements node action)
that splits a source image into a clean background layer, foreground
image artifacts with bounding boxes, and editable text render data.
- New layer_separation.go builds the separated result and artifacts
- SubmitGeneratorTask routes layer-separation and move-object via a
switch; standalone task completion + polling support
- Extend artifact metadata with BBox and GeneratorName across the
domain, API types, mapper, .api schema, and API.md
- edit-elements gets a transparent manual-frame placeholder holding the
separated foreground images and text layers beside the original
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
