Overlap detection for plotter

[viktormai]

Description

This PR adds overlap detection support to the slice data API, enabling
the OpenMC plotter to identify and display which specific cells are
overlapping at a given pixel.

Changes

• Modified check_cell_overlap to return an OverlapResult struct instead
  of a simple boolean, containing a list of OverlapKey pairs (cell1_id,
  cell2_id, universe_id) for each detected overlap
• Added pixel_overlaps_ field to RasterData to store per-pixel overlap
  pair data during raster plot generation
• Added openmc_slice_data_overlap_count C API function to query the number
  of overlapping cell pairs at a given pixel
• Added openmc_slice_data_overlap_info C API function to retrieve the
  overlapping cell IDs and universe ID at a given pixel
• Added corresponding Python bindings in openmc/lib/plot.py

Previously the plotter could color overlap pixels but had no way to tell
the user which cells were overlapping. This change stores the overlap
pair data during plot generation so the plotter can display meaningful
overlap information when a user hovers over an overlap pixel.

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)