[max-models]

• beamtracer.field_map: the max-models field-map format (max-models-field-map-1.0, shared with picnic, whose picnic.input_output.output_format holds the spec and reference validator): FieldMap read/write/from arrays or a function, multilinear interpolation, clip/zero/error outside; validate_field_map, parse_frame_axes; adapters for FieldRegion (3D) and the 2D tracer; a method card for the interpolation (second order in the node spacing).
• BeamTracer takes imposed_electric, a magnetic field in planar geometry, and an azimuthal B in cylindrical geometry.
• FieldMapParams(file, outside) as BeamOpticsConfig.field_map (config round trip); BeamOptics checks the map's frame and dims and adds its E to the solved field and its B to the solenoids or axial field. FieldRegion takes a FieldMap as electric or magnetic.
• In the cylindrical frame the components are axial, radial, azimuthal, as picnic's spec now states.
• Tests: round trip; a map written the way picnic writes one is read correctly and beamtracer writes the same attributes, units and dims; validation; outside handling and maps with fewer dims; a trace through a Gaussian B_z(x) map matches the analytic field; BeamOptics reproduces the Larmor circle and the parabola from uniform maps (rel 1e-5); a cylindrical B map equals an AxialField (1e-9); config round trip.