fix: correct 3d axis tick orientation and avoid duplicate 2d labels

[krystophny]

Summary

• flip the 3D X-axis tick marks and labels so they render beneath the axis in device space
• skip drawing the legacy 2D tick labels when 3D plots are active while still rendering the axis title/x/y labels

Testing

• make verify-artifacts

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https://chatgpt.com/codex/tasks/task_e_68cd9f2942248324b0b3feb6167d67cd

[Copilot]

Pull Request Overview

This PR fixes two issues with 3D plotting: corrects the orientation of X-axis tick marks and labels to render beneath the axis in device space, and eliminates duplicate 2D tick labels when 3D plots are active while preserving axis titles.

• Improved 3D axis tick positioning by considering Y-span direction to determine proper downward orientation in device space
• Replaced legacy 2D axis drawing with dedicated label-only rendering for 3D plots to avoid duplicate tick marks
• Added proper handling of optional string parameters when drawing axis labels in 3D mode

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/plotting/fortplot_3d_axes.f90	Modified tick positioning logic to account for Y-span direction and ensure proper downward orientation
src/backends/raster/fortplot_raster.f90	Replaced 2D axis drawing with label-only rendering for 3D plots and added safe string handling

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