feat(surface): surface deformation prescriptions

restricted the unknown state vector to surface deformation and implemented one prescription, NodalRadialSurface, while the full volumetric displacment field is reconstructed analytically from that. This reduced the number of degrees of freedom in the system by a factor of 80 while also removing many null vectors from the system.
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@@ -24,43 +24,119 @@ Run only the budget and choose its output path with:
./mean_field_experiments --experiment-output gravity_budget.csv --catch2 "[accuracy]"
```
## Stellar-equilibrium null-space experiments
## Reduced stellar-surface conditioning experiments
`stellar_null_space_experiments` is a dedicated diagnostic executable rather
than an ordinary verification or validation test. It constructs the analytic
`n = 3` Lane-Emden seed, probes the three computational translations and three
computational rotations, and compares the Jacobian before and after the strong
centering-row replacement. It records total and residual-block response norms,
the isolated centering contribution, and centered finite-difference errors.
`n = 3` Lane-Emden seed and probes only directions representable by the reduced
surface coordinates: uniform radial homology, three translation-like radial
dipoles, an axisymmetric oblate quadrupole, and a degree-12 zonal spherical
harmonic. Tangential, rotational, stellar-interior-only, and vacuum-only mesh
motions are deliberately absent because they are generated coordinates rather
than root unknowns.
The experiment prints rank-zero progress messages while it builds the seed,
solves its gravity field, and completes each rigid-mode case. Run it with:
solves its gravity field, and completes each surface-mode case. The reachability
probe records the surface-to-volume lift amplification, complete root Jacobian
response by block, and centered-difference agreement at zero and half the
Keplerian angular speed. Run it with:
```text
mpirun -np 1 ./cmake-build-debug-homebrew/stellar_null_space_experiments \
--experiment-output stellar_null_space.csv \
--catch2 "[null_space][rigid_motion]"
--experiment-output reduced_surface_reachability.csv \
--catch2 "[null_space][surface_modes][reachability]"
```
The rotation sweep includes zero rotation and a spherical-state diagnostic at
half the Keplerian angular speed. The rotating result is an operator-symmetry
probe, not a definitive rotating-equilibrium null-space measurement.
The gravity-completed probe solves the linearized mixed gravity subsystem for
the gravity-gradient and gravity-potential variations accompanying each rigid
displacement. It then measures the complete equilibrium response with and
without the centering rows:
the gravity-gradient and gravity-potential variations accompanying each
reduced surface mode. It then measures the complete reduced root response:
```text
mpirun -np 1 ./cmake-build-debug-homebrew/stellar_null_space_experiments \
--experiment-output gravity_completed_null_space.csv \
--catch2 "[null_space][gravity_completed]"
--experiment-output gravity_completed_surface_modes.csv \
--catch2 "[null_space][surface_modes][gravity_completed]"
```
The gravity solver prints its convergence summary, while the experiment prints
the current mode and completed-case count. This probe prepares each rotation
state only once and does not repeat the expensive nonlinear finite-difference
calculations from the original rigid-motion diagnostic.
calculations from the reachability diagnostic.
The surface-frequency probe injects normalized zonal spherical harmonics over
a range of angular degrees. For each degree it lifts the unit surface pattern
once, samples the coefficients of
`det(I + a grad(d))` at the production geometry-inspection points, and locates
the positive and negative critical fractional amplitudes without repeatedly
rebuilding trial geometries. It also records the minimum determinant at
fractional amplitudes `1e-4`, `1e-3`, and `1e-2`:
```text
mpirun -np 1 ./cmake-build-debug-homebrew/stellar_null_space_experiments \
--experiment-output surface_frequency_limits.csv \
--catch2 "[surface_modes][frequency_limit]"
```
The coupled conditioning probe evaluates an extension-aware `n = 3` homology
direction together with the nonuniform reduced surface modes. Its density and
enthalpy tangents include the coordinate-composition terms generated by the
non-affine interior extension, and its gravity variation is completed through
the discrete mixed subsystem so the fixed-infinity exterior response is
consistent. The probe prepares the equilibrium once, reuses one restricted
gravity operator and preconditioner, and uses analytic Jacobian actions:
```text
mpirun -np 1 ./cmake-build-release-homebrew/stellar_null_space_experiments \
--experiment-output coupled_surface_conditioning.csv \
--catch2 "[null_space][surface_modes][conditioning]"
```
The CSV reports prescribed and gravity-completed responses by residual block,
the response normalized by the completed direction, lift conditioning where
applicable, and the convergence of each restricted gravity solve.
The homology mass-cancellation experiment evaluates the signed decomposition
```text
delta M = delta M_density + delta M_geometry
```
without solving gravity or preparing the complete coupled operator. The two
default-build cases provide the registered-order baseline and one uniform
spatial refinement:
```text
mpirun -np 1 ./cmake-build-release-homebrew/stellar_null_space_experiments \
--experiment-output homology_mass_h0_p0.csv \
--catch2 "[null_space][homology][mass_normalization][p_refinement]"
mpirun -np 1 ./cmake-build-release-homebrew/stellar_null_space_experiments \
--experiment-output homology_mass_h1_p0.csv \
--catch2 "[null_space][homology][mass_normalization][h_refinement]"
```
A reproducible one-level uniform polynomial refinement uses a separate build so
all registered field families and their quadrature policies see the same
compile-time order increment:
```text
cmake -S . -B cmake-build-release-homebrew-p1 -G Ninja \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_MAKE_PROGRAM=/opt/homebrew/bin/ninja \
-DCMAKE_C_COMPILER=/opt/homebrew/opt/llvm/bin/clang \
-DCMAKE_CXX_COMPILER=/opt/homebrew/opt/llvm/bin/clang++ \
-DUMFPACK_DIR=/opt/homebrew/lib/cmake/UMFPACK \
-DXAD_DIR=/usr/local/lib/cmake/XAD \
-Dhypre_DIR=/usr/local/lib/cmake/HYPRE \
-Dmfem_DIR=/usr/local/lib/cmake/mfem \
-DBoost_DIR=/opt/homebrew/anaconda3/lib/cmake/Boost-1.82.0 \
-DMEAN_FIELD_UNIFORM_POLYNOMIAL_ORDER_INCREMENT=1
cmake --build cmake-build-release-homebrew-p1 \
--target stellar_null_space_experiments -j 8
mpirun -np 1 \
./cmake-build-release-homebrew-p1/stellar_null_space_experiments \
--experiment-output homology_mass_h0_p1.csv \
--catch2 "[null_space][homology][mass_normalization][p_refinement]"
```
A whole-Jacobian dense singular-value experiment is intentionally deferred.
The checked-in `sandbox.smesh` is too large for a useful dense SVD, and the