Add NEML2 material plugin

[sanathkeshav]

• NEML2 materials ("matmodel": "NEML2") in thermal, small- and large-strain problems: any model compiled with neml2-compile, on CPU or GPU, with history variables and per-voxel or per-phase fields read from the microstructure file.
• Separate plugin library: NEML2 and libtorch never reach the FANS build. The material lives in libfans_neml2.so (-DFANS_NEML2=ON), loaded at run time through a small C interface (include/fans_plugin.h).
• Batched evaluation: plugin materials are evaluated for all their Gauss points before each sweep over the elements.
• pixi: dev-neml2 environment with build-fans_neml2 and neml2-compile tasks.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.