YAML and typed Config

Owner: linstabpy.config.typed. Shared lookup helpers also live in linstabpy.utils._solver_cfg.

Config reads YAML and environment variables. PETSc/SLEPc objects are still created inside the solver modules, but the defaults and fallback choices come from that shared policy layer.

File	Role
typed.py	Typed Config
defaults.py	Numerical defaults, including dissipation policy
schema.py	YAML shape

Backend selection for sparse-direct factor (see docs/COMPUTE_BACKENDS.md):

On the reference direct path (st_solver of mumps/direct/lu) the resolution order is solver.eigen.factor_backend, then LINSTABPY_FACTOR_BACKEND, then the mumps default. The per-solver LINSTABPY_EIGEN_PC_FACTOR_SOLVER_TYPE override is not consulted on that path. It participates only for the iterative eigensolver with pc_type="lu" and for st_solver="strumpack_fgmres" (whose default is strumpack).

An unavailable backend fails a tiny complex PETSc LU probe instead of falling back silently. Default factor is mumps.