3D REPL-based visualization wishlist

This collects a wishlist of plotting features for REPL-based plotting in 3D. One potential path forward is using https://github.com/j-fu/GridVisualize.jl, which provides both cut-planes and isosurfaces.

What quantities do we want to visualize?
- Cut-planes, isosurfaces
- Clipping planes (“cake” slices)
- Visualize mesh on cut-planes etc. (useful for AMR)
- Interactivity through GLMakie

No files, tests, or entry points are named. Start by reviewing the proposed GridVisualize.jl path and GLMakie interactivity, then clarify which quantities and 3D features are required. Done means an agreed, implemented REPL-based visualization scope covering the selected cut-plane, isosurface, clipping, mesh, and interaction needs.

Written by the indexing model from the issue text.