fix(KIM): correct finite-Krook rho-B coefficient

[marjohma]

Summary

• use the derived unit coefficient for the finite-Krook A1 [z0 Z(z0) + 1] contribution to K^{rho B}
• preserve the existing finite-collision and explicit collisionless z0 selection
• add a regression assertion for the complete finite-Krook prefactor
• include no documentation files

Verification

• red: the new finite-Krook assertion failed against the former 0.5 coefficient
• green: test_collisionless_ion_kernel, test_collisionless_fourier_kernel, test_collisionless_ion_assembly, test_susceptibility_collisionless_limit, and test_collision_scale_z0
• result: 5/5 tests passed
• independent code review: no findings

Closes #253

Supersedes #265, which was recreated solely to correct PR authorship.