Store slice overlap indices in cell ID field

[paulromano]

Description

This updates slice plot overlap handling so the encoded overlap index is stored in the cell ID field rather than the material ID field. The Python overlap helper function also now returns a shaped (n, 3) array of overlap metadata and includes docstrings/type annotations.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

All looks good to me @paulromano. Thanks!!