[math-02] Derive kinetic response and Fokker-Planck susceptibilities

[krystophny]

Summary

• derive the FP susceptibility identities and limiting cases independently
• commit deterministic arbitrary-precision oracle values
• add a Fortran oracle consumer covering values, symmetries, and mutation sensitivity

Why

KIM susceptibility functions are checked against an independent symbolic reference rather than implementation self-comparison.

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-193-conventions
Head: fix/kim-fp-194-susceptibility-oracle

Closes #194

[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): #214