feat(libmeanfield): variadic refactor

also added normaliztion operator

extension_example/README.md

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+# MeanField physics extension example
+
+This directory is a small, isolated example for physicists who want to extend
+MeanField without first learning its internal block-matrix machinery.
+
+Start in this order:
+
+1. Read `ideal_gas_radiation.cppm`. It implements a monatomic ideal gas plus
+   equilibrium radiation using the public EOS relation protocol.
+2. Read `rotating_stellar_model.cppm`. It composes that EOS with the existing
+   isobaric surface, fixed-total-mass invariant, and fixed-angular-momentum
+   invariant.
+3. Read and run `demo.cpp`.
+4. Read the tests. They show which claims should be compile-time contracts and
+   which claims require physical or numerical checks.
+5. Use `manual/physics_developer_manual.pdf` as the detailed guide. Its LaTeX
+   source is beside it.
+
+## The important boundary
+
+`makeRotatingStellarModel(...)` produces a valid, strongly typed stellar-model
+specification. The current equilibrium numerical core is still barotropic: it
+expects density to be closed by specific enthalpy alone. An ideal-gas plus
+radiation EOS depends independently on density and temperature, so a complete
+thermal equilibrium solve also needs a temperature or entropy field and its
+governing equation.
+
+The example therefore proves at compile time that model composition succeeds
+and that the present discretizer rejects this model. It does not disguise the
+thermal EOS as a polytrope or claim that a missing energy equation exists.
+
+## Build only this example
+
+From the repository root, configure as usual, then build only these targets:
+
+```sh
+cmake --build cmake-build-profile-homebrew-llvm \
+  --target extension_example_demo extension_example_tests
+```
+
+Run only the extension tests:
+
+```sh
+./cmake-build-profile-homebrew-llvm/extension_example/extension_example_tests
+```
+
+Compile a fresh manual into the build directory:
+
+```sh
+cmake --build cmake-build-profile-homebrew-llvm \
+  --target extension_example_manual
+```
+
+No source under `libmeanfield/` belongs to this example, and the extension test
+executable is separate from the main MeanField regression suite.