chore: pin version to 0.1.1

fix(ci): build sdist only — PyPI rejects raw linux_x86_64 wheel tag

The wheel produced by `python -m build` on Ubuntu ships with
`cp312-cp312-linux_x86_64` as the platform tag. PyPI only
accepts manylinux wheels (auditwheel-repaired inside a
manylinux docker). Properly producing manylinux wheels means
adding cibuildwheel + a docker build step — out of scope here.

Fallback: only upload the sdist. The sdist ships the C++ source
under src/flatrun_native/, so `pip install flatrun` on the
user's machine builds the native extension at install time
using the host's pybind11 + compiler. This is the same model
numpy / scipy use for their source distributions.

`packages-dir: dist/` ensures the action picks up the
single .tar.gz produced by `python -m build --sdist`.