Record a second X-ray wavelength (Kα₁/Kα₂) on constant-wavelength instruments

[AndrewSazonov]

Constant-wavelength instruments can now record a second incident wavelength and its relative intensity — the Cu Kα₁/Kα₂ doublet typical of laboratory X-ray sources. The new wavelength and ratio settings are saved with your project and exported in standard CIF, so the information round-trips when you reload or share a project.

This is a groundwork step: the values are stored and shared but not yet used in pattern calculation, so existing single-wavelength experiments behave exactly as before. Setting only a partial pair (a ratio with no second wavelength) is reported as a clear error rather than being silently ignored.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.38%. Comparing base (813e22d) to head (df804a8).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #209      +/-   ##
===========================================
+ Coverage    90.35%   90.38%   +0.03%     
===========================================
  Files          283      283              
  Lines        24246    24276      +30     
  Branches      2847     2850       +3     
===========================================
+ Hits         21907    21943      +36     
+ Misses        1743     1738       -5     
+ Partials       596      595       -1     

Flag	Coverage Δ
unittests	90.38% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...datablocks/experiment/categories/instrument/cwl.py	100.00% <100.00%> (ø)
src/easydiffraction/io/cif/iucr_transformers.py	90.04% <100.00%> (+1.78%)	⬆️

... and 1 file with indirect coverage changes

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