ENH: group triaxial OPM topomaps by orientation

[PragnyaKhandelwal]

Reference issue (if any)

Closes #13781

What does this implement/fix?

Final fix for #13781: adds caller-facing grouped rendering for colocated triaxial OPM channels so orientation information is shown explicitly across visualization entry points.

Implemented:

• Grouped radial and tangential rendering in Evoked topomap.
• Grouped rendering and connector handling in Evoked joint plots.
• Grouped radial and tangential rendering in ICA component topomaps.
• Regression tests updated for topomap, joint, and ICA triaxial OPM behavior.

Additional information

• Manual visual check done for:

  • Evoked.plot_topomap grouped radial/tangential maps
  • Evoked.plot_joint grouped maps + connector lines
  • ICA.plot_components grouped radial/tangential titles

• A local helper script was used for manual visual smoke-checking on synthetic triaxial OPM data. I can share it if needed.

• Visual outputs:

  • Evoked topomap grouped view:

• Joint plot grouped view:

• ICA grouped components view: