fix(KIM): include harmonic current weight

[marjohma]

Closes #273

Summary

• apply A1 + (1 - m_phi) A2 in FP_G1_j_phi
• apply the same common source coefficient in FP_G1_j_B
• add manufactured checks for both current rows at m_phi = -1, 0, +1
• include no documentation files

Derivation check

The perpendicular reduction gives the common FP source term D_0,m proportional to A1 + (1 - m) A2 once the separately represented finite-Larmor-radius pieces are removed. Charge rows already used this coefficient; the two current G1 rows did not.

Red/green evidence

Before the fix, the new j_phi check at m_phi = -1 returned 0.004374 + 0.008559 i instead of 0.003024 + 0.004779 i. The corrected code passes both current rows for all three harmonics.

Verification

• test_fp_harmonic_prefactors
• test_flr2_fourier_kernel
• test_kim_solver_em
• test_integration_methods

All four pass locally after the commit. Independent review reported no findings.