FXC-5441: speed up monitor postprocessing and improve capacitance accuracy

[momchil-flex]

• Unstructured plotting to use monitor rather than data bounds
• Switch box_clip to sel in symmetry expansion

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Note

Medium Risk
Changes unstructured dataset postprocessing (symmetry expansion clipping and VTK-to-numpy conversion) and TCAD plotting bounds, which could subtly affect returned bounds/visualizations and memory usage.

Overview
Improves unstructured TCAD monitor postprocessing and plotting. Symmetry expansion now clips unstructured datasets using sel_inside() (instead of box_clip()), preserving boundary cells and avoiding over-clipping; tests were updated to assert coverage rather than exact bounds.

Adjusts unstructured mesh/field plotting to use monitor extents. Plot axis limits are derived from the monitor bounds (clipped to the simulation domain) and autoscaling is disabled before overlaying the unstructured mesh, ensuring plots focus on the requested monitor region.

Hardens VTK data ingestion. Converting VTK point arrays to numpy now forces a copy to prevent potential data corruption from views into VTK-managed buffers.

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[Copilot]

Pull request overview

Updates TCAD postprocessing/plotting behavior to use monitor extents (rather than unstructured data extents) for display limits, and adjusts symmetry-expansion trimming for unstructured datasets to improve downstream accuracy (e.g., capacitance).

Changes:

• Compute plot axis limits from the monitor extent (clipped to simulation bounds) for unstructured plot_mesh() / plot_field().
• Replace unstructured box_clip() with sel_inside() during symmetry-expansion trimming.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
tidy3d/components/tcad/data/sim_data.py	Sets plot limits based on monitor bounds for unstructured plotting and changes plotting order/limits behavior.
tidy3d/components/tcad/data/monitor_data/abstract.py	Switches symmetry-expansion trimming for unstructured data from geometric clipping to coverage-based selection.

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Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/tcad/data/monitor_data/abstract.py (100%)
• tidy3d/components/tcad/data/sim_data.py (100%)

Summary

• Total: 21 lines
• Missing: 0 lines
• Coverage: 100%

[momchil-flex]

@dbochkov-flexcompute out of caution I wanted to run this by you one last time: I had to modify the symmetry expansion test, because I also switched to using sel_inside in the symmetry expansion instead of box_clip. I did that for performance for large monitors. The modified test assumes that the original mesh will conform to the symmetry plane, which I think is a fair assumption?