fix: make SAMVG runtime portable

[rasros]

Summary

• select the CPU Transformers pipeline path when CUDA is unavailable
• extract SAMVG runtime policy and shared value types into focused modules
• exercise the samvg extra in CI and release validation
• document that PyPI wheels use the portable renderer fallback

Validation

• ruff check for changed SAMVG files
• pytest tests/refine/test_samvg.py -k sam_runtime -q

No model downloads, CUDA builds, benchmarks, or full-suite runs were performed.