Use `BasisFunctions` object in post-processing

[brendan-m-murphy]

Current we reconstruct the inverse to the basis projections map in post-processing to recover the posterior flux trace on the original lat/lon grid from the traces on the basis regions. We also use this inverse basis map to produce country traces.

With the new BasisFunctions object being created in basis_functions_wrapper, we can use that object to apply these maps instead of re-deriving them. This should simplify the post-processing code.

If the BasisFunctions object is not created in basis_functions_wrapper (since it is currently opt-in), then post-processing should fall back to the existing ("legacy") code.

Deliverable:

• Postprocessing can consume retained BasisFunctions when present.
• If absent, falls back to legacy basis reconstruction.

Key change:

• Prefer BasisFunctions.operator.basis_matrix for flux/country computations when object is available.

Acceptance criteria:

• New and fallback paths produce equal (or tolerance-equivalent) totals on shared test cases.