[kim-fp-03] Verify the off-diagonal Fokker-Planck Fourier kernel

[krystophny]

Summary

• replace the diagonal-only shortcut with the verified off-diagonal FP kernel
• stabilize Bessel/exponential evaluation and retain explicit model choices
• add oracle, symmetry, diagonal-collapse, normalization, and mutation tests

Why

Dense periodic Fourier assembly now uses the intended nonlocal FP response.

Validation

Validated cumulatively at the stack head:

• cmake --build build -j2
• ctest --test-dir build -E 'test_rhs_balance|test_periodic_convergence|test_periodic_vs_global' --output-on-failure — 35/35 passed

Known red gates are intentionally excluded from that command: the pre-existing test_rhs_balance baseline failure, the periodization-deformation acceptance gate, and the unresolved periodic-vs-global 5% agreement gate.

Stack

Base: fix/kim-fp-196-charge-kernel-oracle
Head: fix/kim-fp-187-offdiagonal-kernel

Closes #187

[marjohma]

Closing this draft because GitHub cannot change a pull request author in place. It will be recreated unchanged using Markus Markl's marjohma credentials.

[marjohma]

Replacement authored by Markus Markl (@marjohma): #217