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

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+# 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

examples/portrait.yaml

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+# Portrait pipeline: generate base character → emotion variants → remove bg → upscale.
+#
+# Usage:
+#   vnasset pipeline --config examples/portrait.yaml
+#   vnasset pipeline --config examples/portrait.yaml --force   # re-run everything
+#
+# Output structure:
+#   output/portrait_pipeline/
+#     pipeline.json          ← pipeline summary
+#     base/                  ← stage 1: generated portraits
+#       heroine.png
+#       heroine.json
+#     expressions/           ← stage 2: emotion edits (cross-product)
+#       heroine_smile.png
+#       heroine_smile.json
+#       heroine_angry.png
+#       ...
+#     nobg/                  ← stage 3: background removed (RGBA)
+#       heroine_smile_nobg.png
+#       ...
+#     final/                 ← stage 4: upscaled 2×
+#       heroine_smile_nobg_x2.png
+#       ...
+
+session:
+  sdxl_checkpoint: models/novaAnimeXL_ilV190.safetensors
+  edit_model: models/qwen_image_edit_2509_fp8_e4m3fn.safetensors
+  edit_lora: models/Qwen-Image-Edit-2509-Lightning-4steps-V1.0-bf16.safetensors
+
+output_dir: output/portrait_pipeline
+
+defaults:
+  generate:
+    steps: 20
+    cfg: 4.5
+    negative_prompt: "deformed, ugly, bad quality, lowres"
+  edit:
+    steps: 4
+    cfg: 1.0
+
+stages:
+  # ── Stage 1: Generate base character portrait ──
+  - id: base
+    generate:
+      - id: heroine
+        prompt: "1girl, solo, red hair, glasses, blue eyes, white crop top, standing, portrait"
+        seed: 42
+
+  # ── Stage 2: Edit expression variants (fan-out cross-product) ──
+  - id: expressions
+    edit:
+      input: base
+      prompts:
+        - id: smile
+          text: "make her smile happily with a warm genuine smile"
+        - id: angry
+          text: "make her look angry and furious, furrowed brow"
+        - id: sad
+          text: "make her look sad and crying, tears in her eyes"
+        - id: surprised
+          text: "make her look surprised, wide eyes, mouth slightly open"
+        - id: blushing
+          text: "make her blush intensely, embarrassed expression, pink cheeks"
+
+  # ── Stage 3: Remove backgrounds (batch 1:1 passthrough) ──
+  - id: nobg
+    remove_bg:
+      input: expressions
+
+  # ── Stage 4: Upscale to 2× resolution ──
+  - id: final
+    upscale:
+      input: nobg
+      scale: 2