Add current_kdims for downstream

[ahuang11]

Builds on #161

Expose the key dimensions the plot is currently displaying (active reduction
plus the selected x/y axes) so downstream code can build a holoviews.Dataset
that a selection expression emitted by the plot resolves against, instead of
assuming a fixed reduction or axis order.

Also factors the axis-label → index parsing into a shared _dim_index helper
so current_kdims() and create_plot() stay in sync.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.36%. Comparing base (30c842e) to head (df8bc85).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   77.86%   78.36%   +0.50%     
==========================================
  Files          25       25              
  Lines        1771     1803      +32     
  Branches      218      218              
==========================================
+ Hits         1379     1413      +34     
+ Misses        311      309       -2     
  Partials       81       81              

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[flying-sheep]

nice!