[codex] Add mesh vector calculus functionals

[loliverhennigh]

Summary

• add FunctionSpec-backed mesh LSQ divergence, curl, and double-LSQ Laplacian functionals
• add FunctionSpec-backed cotangent/DEC Laplacian and divergence functionals with Torch references and Warp custom kernels
• wire mesh calculus wrappers and Mesh.divergence/Mesh.curl/Mesh.laplacian through the new functionals while keeping Torch as the mesh-level default for backwards-compatible precision
• add focused functional parity tests and mesh API wiring tests

Validation

• uv run --frozen ruff check physicsnemo/nn/functional/derivatives/_mesh_lsq_operator_utils.py physicsnemo/nn/functional/derivatives/_mesh_cotan_operator_utils.py physicsnemo/nn/functional/derivatives/mesh_lsq_divergence physicsnemo/nn/functional/derivatives/mesh_lsq_curl physicsnemo/nn/functional/derivatives/mesh_lsq_laplacian physicsnemo/nn/functional/derivatives/mesh_cotan_laplacian physicsnemo/nn/functional/derivatives/mesh_cotan_divergence physicsnemo/mesh/calculus/divergence.py physicsnemo/mesh/calculus/curl.py physicsnemo/mesh/calculus/laplacian.py physicsnemo/mesh/mesh.py physicsnemo/nn/functional/init.py physicsnemo/nn/functional/derivatives/init.py test/nn/functional/derivatives/test_mesh_vector_calculus_functionals.py test/mesh/calculus/test_mesh_vector_functional_wiring.py
• uv run --frozen ruff format --check physicsnemo/nn/functional/derivatives/_mesh_lsq_operator_utils.py physicsnemo/nn/functional/derivatives/_mesh_cotan_operator_utils.py physicsnemo/nn/functional/derivatives/mesh_lsq_divergence physicsnemo/nn/functional/derivatives/mesh_lsq_curl physicsnemo/nn/functional/derivatives/mesh_lsq_laplacian physicsnemo/nn/functional/derivatives/mesh_cotan_laplacian physicsnemo/nn/functional/derivatives/mesh_cotan_divergence physicsnemo/mesh/calculus/divergence.py physicsnemo/mesh/calculus/curl.py physicsnemo/mesh/calculus/laplacian.py physicsnemo/mesh/mesh.py physicsnemo/nn/functional/init.py physicsnemo/nn/functional/derivatives/init.py test/nn/functional/derivatives/test_mesh_vector_calculus_functionals.py test/mesh/calculus/test_mesh_vector_functional_wiring.py
• uv run --frozen pytest -q test/nn/functional/derivatives
• uv run --frozen pytest -q test/mesh/calculus

Notes

• Local environment did not expose CUDA, so GPU-specific validation still needs to run in CI/GPU environment.

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