deps: bump cs_util 0.2 -> 0.2.1 so cs_util.size is in the image

pyproject pins cs_util>=0.1.9 and the lock already resolved it from
PyPI, but to 0.2 — the last release *before* the size module. So the
dev image installs cs_util, yet `import cs_util.size` raises
ModuleNotFoundError: every path that would use the size-conversion
helpers is broken in-container.

cs_util 0.2.1 (the current PyPI release, "first release with
cs_util.size") ships cs_util/size.py with the Gaussian size web
(T_to_sigma / sigma_to_fwhm / sigma_to_r50 / ...). `uv lock
--upgrade-package cs-util` bumps only that pin; its dependency set is
unchanged, so no new heavy deps land. After the CI image rebuild
(`uv sync --frozen` in the Dockerfile) `import cs_util.size` resolves.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: cailmdaley