Add a solver backend for an existing domain

[dionhaefner]

Mosaic is designed to grow with community-contributed solver backends. Each domain has room for additional solvers that would broaden the benchmark's coverage.

Candidate solvers (non-exhaustive):

• NS-grid: Taichi (GPU-accelerated, Python)
• Structural-mesh: MFEM (C++, widely used in HPC)
• Thermal-mesh: scikit-fem (pure Python, lightweight FEM)
• All: Tensor-Mesh (JAX-based FEM)

What's involved:
Three files under mosaic/tesseracts/<domain>/<solver-name>/: tesseract_api.py, tesseract_config.yaml, and a Dockerfile (or build_pip_venv.sh). The Add a Solver tutorial walks through the full process using a working example.

Quick check: mosaic run -p <domain> --suites forward -s <your-solver> --debug should complete without errors.