Add mms_part_getspec support-variable override examples

[huangzesen]

Summary

• Add advanced crib examples showing how to pass custom support-data tplot variables to mms_part_getspec.
• Demonstrate mag_name and sc_pot_name for pitch-angle/moment products by copying default FGM/EDP support variables to custom names.
• Demonstrate vel_name with /subtract_bulk by copying the default DIS bulk velocity to a custom name.

Notes

• The implemented spacecraft-potential keyword is sc_pot_name (with underscores), not scpot_name.
• The examples copy the default loaded support variables into custom names, so they demonstrate override syntax without changing the science inputs.

Validation

• git diff --check
• Static checks for mag_name, sc_pot_name, and vel_name examples and expected support-variable copies
• Static source inspection confirmed mms_part_getspec and mms_part_products accept these keywords
• Zhipu/GLM review: no blockers; optional sc_pot_name spelling note applied
• IDL runtime was not available on my PATH, so no local IDL smoke run was performed

Closes #195.