Sparse factor backends

Owner: linstabpy.linalg.

File	Role
factorize.py	Sparse LU / shift-invert factor
backends.py	Backend names, mumps default, probe
shifted.py	Shifted operators
cudss_factor.py	Optional NVIDIA cuDSS path

Requested backends are checked with a tiny complex PETSc LU setup probe. Unavailable backends fail loudly.

Install isolation: one virtual environment per PETSc/SLEPc stack. The CPU MUMPS reference stays .venv. Other stacks use .venv-<stack> and must not delete .venv on --force.