[MaxThevenet]

Warning: very early implementation

Adds a Helmholtz solver for the radiation field of a beam in an undulator, plus the beam push in the undulator and radiation fields. Two modes:

• Envelope: complex slowly varying envelope, undulator-period-averaged beam push (FFT or multigrid solver).
• FullWave: field resolved at the wavelength scale, electron wiggle resolved in the beam push (FFT solver).

When enabled (helmholtz.use_helmholtz = 1), the plasma/beam current deposition and Poisson solves are skipped.
Beamline elements (undulators, thick/thin quadrupoles, phase shifters) are set per beam and applied by the Helmholtz pusher.

Main changes

• src/helmholtz/Helmholtz.{H,cpp} (new): field slices and geometry, Envelope/FullWave solvers, initialization, in-situ diagnostics.
• src/particles/deposition/HelmholtzDeposition.{H,cpp} (new): beam source deposition onto the Helmholtz grid. In envelope mode, arrays jx, jz and rho are hacked and contain refraction and source terms of the Helmholtz envelope.
• src/particles/pusher/HelmholtzParticleAdvance.{H,cpp} (new): beam push with undulator, quadrupoles, phase shifters and radiation field. Shared pusher helpers moved to BeamParticleAdvance.H.
• src/particles/beam/{BeamParticleContainer,MultiBeam}.*: parsing of beamline elements, <beam>.output_ids, beams.tstart_push, pusher dispatch.
• src/Hipace.cpp, src/utils/MultiBuffer.*: integration in the slice loop and MPI transfer of the Helmholtz slices.
• src/diagnostics/*, src/fields/Fields.cpp: helmholtz_diag field diagnostic.
• examples/helmholtz, tests/helmholtz.2Rank.sh (new CI test, both modes, compared with Genesis), docs/.../parameters.rst.

Notes

• The CI test uses lower resolution than recommended (see comments in examples/helmholtz/inputs).
• Envelope mode currently has no natural vertical (y) undulator focusing in the beam push; to be added in a follow-up.
• Some open questions are left as comments in HelmholtzParticleAdvance.cpp (e.g. which time level of the field to push with); to be revisited after merge.