feat(cli, demo): add backend decision log, polish info, and add CPU-only E2E demo test (fast, no net)

[3-0-0]

Summary

• expand backend detector with detailed reasons, Vulkan check, and Pillow fallback
• add demo upscale command group and richer info output with platform and package details
• document canonical backends and env overrides; add fast CPU-only demo test and wire it into CI

Testing

• ruff check src/scaleforge/backend/detector.py src/scaleforge/cli/demo.py src/scaleforge/cli/info.py src/scaleforge/cli/__init__.py src/scaleforge/cli/main.py src/PIL/__init__.py tests/test_demo_e2e.py
• pytest -q tests/test_demo_e2e.py
• python -m scaleforge.cli detect-backend --debug
• python -m scaleforge.cli info

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https://chatgpt.com/codex/tasks/task_e_68a5a2d93878832b8fd2018eff0b263c