Add physical-plane FEM model slice diagnostics and plots

experiments/model_slice_geometry_tests.cpp

@@ -0,0 +1,13 @@
+// Standalone, no MFEM or MPI setup: c++ -std=c++20 -O0 this_file.cpp -o /tmp/slice_checks
+#include "model_slice_geometry.hpp"
+#include <iostream>
+
+int main() {
+    try {
+        for (const auto& check : experiment::model_slices::GeometrySelfChecks())
+            std::cout << check.name << ": " << check.observed << '\n';
+    } catch (const std::exception& error) {
+        std::cerr << error.what() << '\n';
+        return 1;
+    }
+}