Refactor Fock basis construction

[lajerm]

This refactors Fock-basis handling so that model setup returns compact
sector metadata rather than materializing the full basis.

Main changes:

• move explicit and combinadic basis representations into fock_basis.py
• defer basis realization to operator construction
• support explicit/combinadic basis routes
• support vanilla/Numba operator construction, with Numba opt-in
• preserve historical EDRIXS basis ordering in the combinadic route
• separate backend-independent operator kernels from backend-specific
  sparse-matrix assembly
• retain the existing scipy_edrixs PETSc stub without depending on the
  implemented PETSc backend
• add numerical consistency tests for SciPy, explicit/combinadic, and
  vanilla/Numba routes

The backend-independent portions correspond to PR #278, adapted minimally
for the scipy_edrixs branch.

Local test result: 383 passed