add YAML-driven pipeline batch mode (vnasset pipeline)

Multi-stage pipelines declared in YAML: generate, edit (cross-product),
remove_bg, and upscale stages executed sequentially in a single GPU
session. Every intermediate artifact (image + metadata JSON) is saved
to {output_dir}/{stage_id}/ for full traceability.

Data routing:
- generate: list of prompts → list of images (1:1 per item)
- edit: fan-out cross-product (each input × each prompt)
- remove_bg / upscale: 1:1 passthrough

Resume: outputs that already exist are skipped. Use --force to re-run
everything — lets you add items to a stage without regenerating.

Examples: examples/portrait.yaml, examples/backgrounds.yaml

examples/backgrounds.yaml

@@ -0,0 +1,49 @@
+# Background batch: generate multiple background images from independent prompts.
+#
+# Usage:
+#   vnasset pipeline --config examples/backgrounds.yaml
+#
+# Output structure:
+#   output/backgrounds_pipeline/
+#     pipeline.json
+#     backgrounds/
+#       classroom.png
+#       classroom.json
+#       hallway.png
+#       hallway.json
+#       ...
+
+session:
+  sdxl_checkpoint: models/novaAnimeXL_ilV190.safetensors
+
+output_dir: output/backgrounds_pipeline
+
+defaults:
+  generate:
+    steps: 20
+    cfg: 4.5
+    negative_prompt: "deformed, ugly, bad quality, lowres"
+    width: 1024
+    height: 576
+
+stages:
+  - id: backgrounds
+    generate:
+      - id: classroom
+        prompt: "anime classroom interior, empty, sunlight through windows, desks and chairs, blackboard, warm atmosphere"
+        seed: 100
+      - id: hallway
+        prompt: "anime school hallway, lockers, windows on one side, afternoon light, clean floor"
+        seed: 101
+      - id: park
+        prompt: "anime park, cherry blossoms, green grass, bench, path, spring afternoon, peaceful"
+        seed: 102
+      - id: rooftop
+        prompt: "anime school rooftop, blue sky, chain link fence, water tower, sunset colors"
+        seed: 103
+      - id: cafe
+        prompt: "anime cafe interior, cozy, warm lighting, wooden tables, counter with pastries, quiet afternoon"
+        seed: 104
+      - id: street
+        prompt: "anime city street, shops, pedestrians, evening, neon signs starting to glow"
+        seed: 105