Warn when input boundary is spectrally dense

[jurasic-pf]

Fixed boundary runs can produce poor results, when the boundary has high spectral content. This can commonly be resolved with spectral condensation.
A heuristic that detects if this might become a problem and emits a corresponding warning message, would be helpful to users.

[CharlesCNorton]

Addressed by #600. IsConsistent now computes the input boundary's spectral width <M> (the wout.specw / FourierGeometry::ComputeSpectralWidth quantity, evaluated on the input rbc/zbs/rbs/zbc) and warns, before the solve, when a fixed-boundary input exceeds a threshold.

The threshold (6) is calibrated from ripple sweeps on a tokamak and a 3D stellarator: fixed-boundary runs converge for <M> below ~6 and fail to initialize from ~7.2, while all bundled inputs stay at <M> <= 1.75. So the warning fires on the dense boundaries that actually fail and points at spectral condensation, instead of the user only seeing the late, generic "isn't spectrally condensed enough" abort after the solve has started.