Hybrid-PIC: keep thermal relaxation momentum neutral

[tomzhu0225]

Summary

This keeps the Hybrid-PIC electron-ion temperature-relaxation operator thermal-only by centering its ion Ornstein-Uhlenbeck update on each ion species' bulk velocity instead of the electron velocity.

This PR is stacked on BLAST-WarpX/warpx#7103 because it reuses the per-species Vs_fp_<species> fields introduced there. It can be rebased and retargeted to development after that PR merges.

Problem

The current QDSMCApplyIonHeating update relaxes each ion velocity toward u_e. However, the paired electron-energy term is

Q_ei = 3 n_i k_B nu_ei (T_e - T_i)

and accounts only for thermal-energy exchange. Damping an electron-ion bulk drift also transfers ion momentum and bulk kinetic energy, without a conjugate electron momentum or energy update. Bulk friction is separately represented by the resistive-drag operator in BLAST-WarpX#7103.

Changes

• Deposit and refresh per-species ion fluid velocities when temperature relaxation is enabled.
• Center the ion thermal update on u_i from Vs_fp_<species>.
• Convert cylindrical (u_r, u_theta, u_z) moments back to Cartesian particle velocity components in RZ and RCYLINDER.
• Clarify the thermal-only, momentum-neutral contract in the parameter and model documentation.
• Extend the Qei regression with a force-free electron-ion drift.

Regression

The enhanced test uses a force-free magnetic field, so J || B supplies an electron-ion drift without a J x B force, while eta = 0 disables Joule heating.

With this fix:

• fitted temperature-difference rate error: 0.97% (tolerance 5%)
• total thermal-energy drift: 0.603% (tolerance 2%)
• max projected ion current: 0.0534 J0 (tolerance 0.08 J0)

Negative control using the same test on the unmodified BLAST-WarpX#7103 head:

• thermal-rate and energy checks still pass
• max projected ion current reaches 0.3817 J0
• the expected bulk-relaxing response is approximately 0.3297 J0

Local checks

• Targeted 2D MPI/OMP/Python/openPMD Qei run and analysis pass.
• The negative-control analysis fails only the new ion bulk-momentum check.
• Cartesian 2D, RZ, and RCYLINDER OMP application builds pass.
• Python Ruff check and format check pass.