Add experimental support for exporting mplot3d plots

[DanielBetschinske]

Summary

This PR adds experimental support for exporting Matplotlib mplot3d plots to PGFPlots/TikZ.

Supported so far:

• 3D line plots
• 3D scatter plots
• surface plots
• wireframe plots
• contour plots
• bar3d plots
• quiver3d plots

I opened this as a draft because it's the first implementation of a relatively large change that touches core export logic. Since the supported 3D plot types share common infrastructure, I am not sure whether splitting this into smaller PRs would be useful. Before marking it as ready for review, I would be happy to get feedback on whether the overall approach looks reasonable or if there are major changes I should make first.

Motivation

The README currently states that 3D plots do not work. This PR closes that gap with a first comprehensive implementation.

Design notes

• Adds dedicated 3D handling in the export pipeline.
• Preserves existing 2D behavior where possible.
• Adds clipping support where required for the new 3D export paths.
• Adds reference tests for the supported 3D plot types.
• Uses PGFPlots \addplot3 where possible.

Known limitations / open points

• Occlusion is not handled by PGFPlots. Some cases may require splitting plots by depth/z-layer in the future.
• The current implementation focuses on common 3D plot types and tested reference cases; it likely does not cover all edge cases yet.
• Some 3D Matplotlib artists only expose their data through private attributes, so parts of the implementation depend on Matplotlib implementation details.