TST: rewrite CI around uv.lock

[neutrinoceros]

Context: numpy 2.2.0 just dropped and CMasher does not typecheck against it. Not all errors seem genuine and I may not have time to address/report all of them soon, so I'm stabilizing CI with a lock file.

[neutrinoceros]

I hit a very weird failure mode in numpy

 ImportError while loading conftest '/home/runner/work/CMasher/CMasher/conftest.py'.
conftest.py:3: in <module>
    import matplotlib as mpl
../../_temp/setup-uv-cache/archive-v0/WvtTXssC9sSStd9mylBSV/lib/python3.10/site-packages/matplotlib/__init__.py:159: in <module>
    from . import _api, _version, cbook, _docstring, rcsetup
../../_temp/setup-uv-cache/archive-v0/WvtTXssC9sSStd9mylBSV/lib/python3.10/site-packages/matplotlib/cbook.py:24: in <module>
    import numpy as np
../../_temp/setup-uv-cache/archive-v0/WvtTXssC9sSStd9mylBSV/lib/python3.10/site-packages/numpy/__init__.py:135: in <module>
    raise ImportError(msg) from e
E   ImportError: Error importing numpy: you should not try to import numpy from
E           its source directory; please exit the numpy source tree, and relaunch
E           your python interpreter from there.

I may need to finish #145 first to match my repos where this locking strategy was applied successfully.