### Summary

`setup-macos.sh` runs `install_pip_dependencies` before
`install_pytorch_and_domains`. That order lets torchsr's transitive
torch dep get pulled from PyPI before the pinned source-built or
S3-cached torch lands; `install_pytorch_and_domains` then overwrites the
wrong-source torch.

The overwrite is small in the current state, but the same race forced
--no-cache-dir`. That cascaded reinstalls of every torch transitive dep,
pushed the macOS unittest past its 60-minute timeout, until #19334
reverted it.

Reorder so `install_pip_dependencies` runs after
`install_pytorch_and_domains`. With torch already at the pinned version,
torchsr's dep is satisfied and pip skips re-downloading. Removes the
structural footgun so any future re-land of centralized torch install
does not need `--force-reinstall`.

Also rewrite two adjacent comments:
- Add a comment above `install_pytorch_and_domains` recording the
ordering rationale.
- Drop the now-misleading "We build PyTorch from source here" comment
that drifted above `install_executorch`; replace with one explaining why
`install_executorch --use-pt-pinned-commit` is correct (torch is already
installed).

Authored with Claude Code.

### Test plan
CI