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
• let SciPy and PETSc independently control their matrix-construction
  and assembly policies
• use bounded work ranges for PETSc construction instead of materializing
  the complete local sparse matrix before insertion
• keep PETSc assembly policy isolated so that improved preallocation,
  MPI ownership, and GPU-specific construction can be added later
• add numerical consistency tests across SciPy/PETSc,
  explicit/combinadic, and vanilla/Numba routes

Local test result:
389 passed, 3 skipped

[mpmdean]

I'm happy for this to be merged.