Reject unsupported periodic collision models

[krystophny]

The periodic-Fourier driver currently assembles the restored Krook kernels regardless of collision_model. Reject every model except Krook until a continuum Fokker-Planck kernel is implemented, so an input cannot silently claim different collision physics.

Invariant: the supported Krook periodic assembly, Fourier convention, CGS units, grid, dense solve, and boundary behavior are unchanged.

Verification

Failing before, using the parent binary with collision_model = FokkerPlanck:

parent_exit=0
Collision Model............. FokkerPlanck
...fourier_periodic solve finished with 257 modes.

Passing after, using the same input:

fixed_exit=1
ERROR STOP fourier_periodic: only collision_model = Krook is implemented

100% tests passed, 0 tests failed out of 30
Static: OK
Build: OK
Tests: OK
Lint: OK
All stages passed

Relation to Markus's Fokker-Planck periodic branch

This guard closes a real semantic hole in the independent stack: a Krook kernel must not run under a Fokker-Planck label. Keep the guard until #187 verifies the FP kernel and #190 accepts the FP benchmark. #191 must replace the blanket rejection with explicit Krook/FP dispatch, not remove collision-model validation.