Upgrade FEniCS solvers to FEniCSx (legacy FEniCS 2019.1.0 pins us to Python 3.10)

[dionhaefner]

The two FEniCS-based Tesseracts pin the legacy fenics==2019.1.0 / dolfin-adjoint==2019.1.0 stack:

• mosaic/tesseracts/thermal-mesh/fenics-heat/tesseract_environment.yaml
• mosaic/tesseracts/structural-mesh/fenics-structural/tesseract_environment.yaml

dependencies:
  - python==3.10
  - fenics==2019.1.0
  - dolfin-adjoint==2019.1.0

Why this matters

Legacy FEniCS (fenics/dolfin 2019.1.0) has been superseded by FEniCSx (dolfinx) and is no longer actively developed. It has no conda-forge builds for Python 3.12+, which forces these two solvers to pin python==3.10.

Python 3.10 reaches end-of-life in October 2026 and already lags the rest of the project — the main package and mosaic-shared target newer interpreters. As long as these solvers depend on legacy FEniCS, the whole repo is held back to the 3.10 floor.

The task

Port both solvers from legacy FEniCS to the modern FEniCSx stack:

• fenics → fenics-dolfinx (conda-forge, has 3.12/3.13 builds)
• dolfin-adjoint → adjoint via pyadjoint / the FEniCSx adjoint path
• Bump python==3.10 → 3.12 in both tesseract_environment.yaml files

The tesseract_api.py in each solver directory will need updating for the DOLFINx API (function spaces, forms, assembly, and the adjoint tape all changed between the two generations).

Acceptance criteria

• Both fenics-heat and fenics-structural Tesseracts build and run on Python 3.12
• Forward + gradient (VJP) outputs match the current legacy implementation within tolerance
• python==3.10 no longer appears in either environment file

Pointers

• FEniCSx / DOLFINx docs: https://docs.fenicsproject.org/
• Adjoint (pyadjoint): https://www.dolfin-adjoint.org/
• Reference implementations in this repo already use adjoint AD — see the dolfin-adjoint tape logic in each solver's tesseract_api.py.

Context: surfaced while reverting a Python-version bump (#96); the legacy FEniCS pin was the blocker for moving off 3.10.