Port Python bindings to clair + c2py and sync app4triqs skeleton

[Wentzell]

Ports TPRF's Python bindings to clair + c2py and syncs the latest app4triqs skeleton.

Changes

• Migrate Python binding generation from cpp2py (*_desc.py) to clair + c2py (.toml + .cpp + generated .wrap.*) for all modules (lattice, linalg, chi_from_gg2, freq_conv, hubbard_atom)
• Convert C++ header docstrings from RST to doxygen style
• Fix doc-comment defects surfaced by the port: RST→doxygen math blocks, 13 @param name mismatches, restored missing formula clauses, CPP2PY_IGNORE→C2PY_IGNORE

app4triqs skeleton merge (unstable)

• Full doxygen C++ API docs (drops the legacy cpp2rst path and doc/themes/)
• k8s-based Jenkins setup (jenkins/Jenkinsfile + jenkins/Dockerfile)
• CI toolchain for clair-c2py (clang-20/llvm-20, clair build step, regenPlatforms), CMAKE_CXX_SCAN_FOR_MODULES OFF, ENVIRONMENT_MODIFICATION for PYTHONPATH, version bump to 4.0.0

Testing

• Builds clean (clang)
• ctest: 63/65 pass — the 2 failures (Py_eliashberg DLR) are pre-existing and unrelated to this branch

Note

⚠️ Jenkins failure-notification recipients are no longer set inline in the new jenkins/Jenkinsfile (handled centrally by the new k8s Jenkins) — please confirm routing.

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