Fix AlignmentError in to_xarray_dataset when data variables have different setpoints

[Copilot]

DataSet.to_xarray_dataset() raises an AlignmentError when two data variables share a setpoint coordinate but one of them has additional non-grid setpoints (triggering the multi-index export path).

Root cause: Per-parameter sub-datasets are built independently. A non-grid array parameter (e.g. spectrum with setpoints (f_stop, freq) where freq values differ per sweep point) gets exported via xr.Coordinates.from_pandas_multiindex, making f_stop a component of a MultiIndex rather than a standalone dimension. A co-registered scalar parameter with only (f_stop,) gets a plain PandasIndex for f_stop. xr.merge cannot align these incompatible Index objects.

meas.register_parameter(a.spectrum, setpoints=(a.f_stop,))  # array, non-grid → multi_index path
meas.register_parameter(a.bar, setpoints=(a.f_stop,))       # scalar → standalone f_stop dim

dataset.to_xarray_dataset()
# AlignmentError: cannot align objects on coordinate 'foobar_f_stop'
# because of conflicting indexes

Fix (export_to_xarray.py):

• Before xr.merge, detect sub-datasets that use a multi_index dimension whose coordinate levels conflict with standalone dimensions in other sub-datasets
• Resolve by calling ds.unstack("multi_index") on the conflicting sub-datasets, converting to regular independent dimensions (NaN-filling off-grid entries) so all sub-datasets share a consistent f_stop coordinate structure

Test (test_dataset_export.py):

• Regression test sweeping f_stop over two values with differing frequency grids, verifying both spectrum (2-D, NaN-padded) and bar (1-D) are correctly present with values preserved in the merged dataset

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.03%. Comparing base (a095809) to head (59d0ba4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8244      +/-   ##
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+ Coverage   71.02%   71.03%   +0.01%     
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  Files         301      301              
  Lines       31888    31900      +12     
==========================================
+ Hits        22647    22659      +12     
  Misses       9241     9241              

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