fix: add 2pi² to z-state diffusion computation

[J-Endres]

the diffusion computation for z-states was missing a 2pi when computing the b-value from the k-dephasing. k has unit 1/m while the formula expects rad/m. For + states this error was not present.

The effect is minor for most sequences.

[github-actions]

Simulation test: main (before) vs. this PR (after)

Sequence	Status	max mag NRMSE	max phase RMSE	max runtime Δ
mr0_DREAM_STE_seq	❌	3.05e-02	6.05e-01	+0.8%
mr0_DWI_SE_EPI	✅	0.00e+00	0.00e+00	+2.2%
mr0_EPI_2D_seq	✅	0.00e+00	0.00e+00	-1.6%
mr0_FID_seq	✅	0.00e+00	0.00e+00	+1.3%
mr0_FLASH_2D_seq	❌	7.10e-02	5.74e-01	+4.7%
mr0_GRE_to_FLASH	⚠️	–	–	–
mr0_RARE_2D_seq	❌	3.20e-01	4.67e-01	+2.3%
mr0_SE_CPMG_seq	✅	0.00e+00	0.00e+00	+1.1%
mr0_STE_3pulses_5echoes_seq	✅	6.86e-09	1.49e-08	+4.2%
mr0_TSE_2D_multi_shot_seq	❌	2.42e-02	1.90e-01	+1.6%
mr0_bSSFP_2D_seq	✅	0.00e+00	0.00e+00	+5.6%
mr0_diffusion_prep_STEAM_2D_seq	❌	1.64e-02	2.86e-01	+9.0%

❌ mr0_DREAM_STE_seq: Mag RMSE too high (0.03046) at acc=0.50000

✅ mr0_DWI_SE_EPI: All tests passed.

✅ mr0_EPI_2D_seq: All tests passed.

✅ mr0_FID_seq: All tests passed.

❌ mr0_FLASH_2D_seq: Phase RMSE too high (0.11352) at acc=0.10000

⚠️ mr0_GRE_to_FLASH: No comparison possible: [Errno 2] No such file or directory: '/home/runner/work/MRzero-Core/MRzero-Core/tests/simulation_test/ref_files/mr0_GRE_to_FLASH.seq.npz'

❌ mr0_RARE_2D_seq: Mag RMSE too high (0.01109) at acc=0.60000

✅ mr0_SE_CPMG_seq: All tests passed.

✅ mr0_STE_3pulses_5echoes_seq: All tests passed.

❌ mr0_TSE_2D_multi_shot_seq: Mag RMSE too high (0.02420) at acc=0.01000

✅ mr0_bSSFP_2D_seq: All tests passed.

❌ mr0_diffusion_prep_STEAM_2D_seq: Mag RMSE too high (0.01070) at acc=0.60000

Workflow run — raw .npz data is attached as artifacts.