Improve VMEC from_cyl Newton iteration robustness

[krystophny]

Priority: High

Problem

The VMEC from_cyl Newton iteration in libneo can fail for points near the magnetic axis or outside the plasma domain. This causes silent failures or crashes during coordinate transformations, particularly in:

• Chartmap spline construction (when sampling VMEC field at chartmap grid points)
• Particle position conversions between coordinate systems

Current Behavior

• Newton iteration may not converge
• Error handling is inconsistent across call sites
• Some failures are silent, others cause crashes

Proposed Solution

1. Add robust error handling at all from_cyl call sites
2. Implement fallback strategies (e.g., nearest valid point, graceful particle loss)
3. Consider adding iteration limits and convergence tolerance parameters
4. Document which regions are problematic (near axis, near/outside separatrix)

Related

• Chartmap boundary limitation (splines restricted to [0.1, 0.99] to avoid this issue)
• See DOC/coordinates-and-fields.md Appendix A.4