Regularize JOREK axis field evaluation

[krystophny]

Summary

• detect collapsed magnetic-axis edges while building the spatial locator
• evaluate the finite model-303 axis limit from two stable off-axis radii
• angularly weight nonuniform axis sectors and preserve the exact F0/R toroidal field
• cover the collapsed-axis behavior with an analytic reduced-MHD fixture

The off-axis Hermite field, Newton locator, component ordering, and units are unchanged. At the coordinate singularity, the implementation extrapolates the physical flux gradient before applying the exact axis radius. The extrapolation uses normalized element radii 1e-2 and 1e-3; the analytic axis regression uses a relative tolerance of 1e-12.

Verification

Red before the implementation:

global model 303 query has a finite collapsed-axis limit
 ierr mismatch
 axis B_R mismatch
 axis B_Z mismatch
 axis B_phi mismatch
 FAIL

Green after the implementation:

• fo
  • Static: OK (228 modules, 228 changed, 228 affected)
  • Build: OK
• ctest --test-dir /tmp/libneo-axis-build -R jorek --output-on-failure
  • 100% tests passed, 0 tests failed out of 5
• full model-303 restart (6,129 elements, 115 collapsed axis elements)
  • finite axis evaluation at two toroidal phases
  • B_phi = F0/R_axis exactly
  • regularized poloidal field agrees with the convergent off-axis limit