The pinned-index work landed for install.sh and install_python_stack.py, but the
Windows installers still picked the wheel index from GPU probing. Extend the same
UNSLOTH_TORCH_INDEX_URL / _FAMILY contract so a pinned index wins on every platform:
- install.ps1: Get-TorchIndexUrl returns the pinned URL/family before nvidia-smi
probing; the AMD ROCm reroute is skipped when the index is pinned, so an explicit
cpu/cu* pin on an AMD host is not overwritten.
- studio/setup.ps1: add shared Get-PinnedTorchIndexUrl / Get-TorchIndexLeaf helpers;
the stale-venv check, the install selection and the AMD reroute all honor the pin,
and the CPU/CUDA install pulls from the resolved index URL.
- tests: parity test that all four installers read both override vars and the two
Windows installers gate the AMD reroute on the pinned flag.
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