Add parallel flow to x2 in KIM-FP

[krystophny]

Summary

• add toroidal-flow profile input and configuration plumbing
• include the parallel-flow contribution in the FP susceptibility argument x2
• verify zero-flow compatibility, sign behavior, interpolation, and periodic-background handling

Why

The FP response now represents the configured plasma flow instead of silently omitting its parallel component.

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-02-multispecies-background
Head: fix/kim-fp-37-parallel-flow

Closes #37

[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): #213