[lachlangrose]

Three problems with solver='native' that showed up in the QGIS plugin with a real model (six faults and one stratigraphic group).

1. Structural frames (faults) can use the native solver

The coordinates 1 and 2 of a structural frame get gradient orthogonal constraints (add_orthogonal_feature → add_gradient_orthogonal_constraints). These were built as sparse rows after the setup, so check_supported refused them (add_constraints_to_least_squares), and every fault fell back to cg.

With b = 0 an orthogonal constraint is a tangent constraint (gradient · vector = 0), which the core already supports. Now, in a native setup, FiniteDifferenceInterpolator.add_gradient_orthogonal_constraints keeps the points, the vectors and the weight as raw data (_native_orthogonal, cleared by reset), and _native._add_data gives them to the core with add_tangent. A constraint with b != 0 still builds rows, so the check still refuses it.

2. The native iteration limit follows the grid

The core stops after 5000 iterations by default. cg (scipy) allows 10 × the number of unknowns. On a grid of 87 232 nodes the native solve stopped at 5000 iterations with a residual of 1.2e-6. Now, without maxiter in solver_kwargs, _core_settings uses 10 × n_nodes. With it the same model converged in 7442 iterations (4.6 s).

3. No stale field after a solve that does not converge

When a native solve did not converge, c kept its old value. If the grid changed after the last solve (for example a new number of elements), the old c had the old size, the builder still marked the feature as built, and the surfaces failed with cannot assign 51000 vlaues to array of shape 87232. Now, if the old c is missing, only NaN, or from another grid, c gets the partial field (as the matrix solvers do). A valid old field of the same grid is still kept, as before.

Tests

• New packages/loop_interpolation/tests/test_native_orthogonal_and_limits.py: orthogonal constraints are kept as raw data and give the same field as cg (within 1e-4 of the range); b != 0 is still refused; the default limit; a non-converged solve replaces a field of another grid and keeps a field of this grid.
• New tests/unit/modelling/test_native_fault_frame.py: all three coordinates of a fault use native and converge, and the frame matches the cg frame (largest difference 9e-5 of the range). For that fault, native takes 3.7 s and cg 12.5 s.
• packages/loop_interpolation/tests: 655 passed. tests/unit: 709 passed (with NumPy 2.4; with NumPy 2.5 the 2D tests in test_2d_p1_p2_support.py fail because of np.cross, as noted in feat: allow loop-interpolation 0.2 and add the native extra #381).

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