feat(server): add layer-separation generator task

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>

server/API.md

@@ -238,6 +238,25 @@ Returns the updated project, the assistant message, and generated nodes.
 
 The generation path uses the configured design agent runner. The default is the `sky-valley/pi` Go agent adapter, with local desktop/MCP agent behavior disabled.
 
+## Generator Tasks
+
+`POST /api/generator/tasks`
+
+```json
+{
+  "cid": "client-request-id",
+  "project_id": "project-id",
+  "generator_name": "layer-separation",
+  "input_args": {
+    "image_url": "https://cdn.example.com/source.png"
+  }
+}
+```
+
+`layer-separation` returns a task id that can be polled with `GET /api/generator/tasks?task_id=<id>&project_id=<project-id>`. Completed tasks return a clean background image, foreground image artifacts with `bbox`, and a `text_render_data` artifact for editable text reconstruction.
+
+The canvas `edit-elements` node action uses the same separation path but writes the separated result beside the original image. The original image remains unchanged; the clean background is a bottom image layer, and a transparent frame above it holds the separated foreground images and editable text layers.
+
 ## Chat History Replay
 
 `GET /api/projects/:id/history`