Two failures from the first in-image Studio install, both rooted in
install.sh probing the build host:
1. setup.sh aborted on the pre-linked llama.cpp dir: 'already exists and
is not marked as a Studio-owned llama.cpp install'. The dir is the
image's baked prebuilt, provisioned exclusively for Studio, so write
the .unsloth-studio-owned marker next to the binaries.
2. With no GPU and no nvidia-smi in the build container, install.sh fell
back to cu126 torch wheels for the Studio venv (and would pick cpu
wheels on a CI runner without /proc/driver/nvidia), so the published
image's Studio venv would depend on which host built it and could not
train on Blackwell. get_torch_index_url now honours an explicit
UNSLOTH_TORCH_INDEX_FAMILY override naming the index leaf (cu128,
cu130, rocm7.2, cpu, ...). The resolved family flows into
UNSLOTH_TORCH_BACKEND, which install_python_stack.py already consumes,
so the whole downstream chain follows the pin. Dockerfile.studio sets
cu128 on amd64 and cu130 on arm64 (DGX Spark / Grace).
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