Add spatial hash describe function

[wyatt-fluidnumerics]

Description

Currently, it is difficult to get a sense for many of the dimensions of the spatial hash table that gets built over unstructured, and many structured grids. This makes understanding the performance speed up due to spatial hashing, or potential bugs in the spatial hashing such as that addressed by PR #2780, difficult. This PR adds functionality for .describe() to the SpatialHash class. The function is used exclusively for debugging or logging and provides useful statistics regarding the size of the hash grid and the ratio of hash grid cells to mesh grid faces.

Checklist

• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
  • Describe how you used it (e.g., by pasting your prompt): I used Claude Code to generate an initial implementation of the spatialhash_describe function. I then made significant modifications to the resulting code to improve the utility of the reported statistics.