feat(libmeanfield): centrifugal + pressure

experiments/README.md

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+# Accuracy experiments
+
+This directory is deliberately separate from `tests/`. It uses Catch2 only as
+an MPI-aware runner, selector, configuration host, and failure reporter. A run
+fails only when the calculation is invalid (for example, non-finite output or a
+failed linear solve); measured accuracy is written as data, not as a regression
+tolerance.
+
+`experiment_main.cpp` registers the `experiment` Catch2 reporter. It collects
+rows recorded with `record_experiment_result` and writes one wide CSV table on
+MPI rank zero. Each row carries sweep parameters and numerical metrics, so
+results can be plotted or compared across commits.
+
+The initial budget has three sweeps at fixed mesh and polynomial orders:
+
+1. linear-solver tolerance: `1e-8`, `1e-10`, `1e-12`, `1e-14`;
+2. production quadrature boost: `0`, `4`, `8`;
+3. reference-space decomposition: numerical field/potential error, analytic
+   projection error, and numerical-to-projection gap.
+
+Run only the budget and choose its output path with:
+
+```text
+./mean_field_experiments --experiment-output gravity_budget.csv --catch2 "[accuracy]"
+```
+
+The executable needs the same dependencies, generated module mapping, and
+configuration registration as the existing Catch2 test executable. Add
+`experiment_main.cpp` and `gravity_accuracy_budget.cpp` as a second executable
+next to that target; do not add them to the ordinary test executable.