[bloch] solve transverse magnetic flux with a consistent energy metric

docs/api/fdfd.md

@@ -14,6 +14,25 @@
 
 ::: meanas.fdfd.farfield
 
+## Bloch magnetic materials
+
+Bloch state vectors store the transverse Fourier coefficients of magnetic flux
+`B = mu * H`. They equal `H` when permeability is omitted. Pass the same `mu`
+to `bloch.hmn_2_hxyz(...)` and `bloch.hmn_2_exyz(...)` when converting solved
+vectors to fields. The eigensolver uses the projected `mu**-1` energy metric
+for finite positive real permeability, including anisotropic and spatially
+varying profiles. Its preconditioner remains finite at the Gamma point.
+
+## Far-field normalization
+
+The near-to-far transform includes the cell area `dx * dy`, so normalized
+far-field amplitudes describe a Fourier integral and converge under spatial
+refinement. This changes amplitudes previously returned as unweighted FFT sums.
+The inverse applies the matching Fourier quadrature and returns spacings for
+the requested output shape. Both transforms center their origins at index
+`size // 2`; padding and cropping preserve that sample. The inverse does not
+modify input spectra.
+
 ## SCPML conventions
 
 `stretch_with_scpml` now uses `-1` for the low-index face and `+1` for the