performance optimization of stats code for rank_genes_groups

[zboldyga]

• Closes #
• [x ] Tests included or not required because:

• Release notes not necessary because:

@ilan-gold replayed changes of #4118 onto main

[zboldyga]

@ilan-gold regarding these points, perhaps I open separate PR(s):

• addressing the scanpy 2.0 change in how cells with group NaN/unassigned are handled (uniform treatment across aggregate, rank_genes_groups, anywhere else relevant?, + docs update for this).

• adding benchmark code for rank_genes_groups paths that are fast-enough to run quickly (e.g. < 1 min).

[codecov]

Codecov Report

❌ Patch coverage is 97.16981% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.93%. Comparing base (eea0015) to head (304a1b5).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/tools/_rank_genes_groups.py	97.16%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4204      +/-   ##
==========================================
+ Coverage   79.90%   79.93%   +0.03%     
==========================================
  Files         121      121              
  Lines       12949    12976      +27     
==========================================
+ Hits        10347    10373      +26     
- Misses       2602     2603       +1     

Flag	Coverage Δ
hatch-test.low-vers	79.06% <97.16%> (+0.03%)	⬆️
hatch-test.pre	79.90% <97.16%> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/tools/_rank_genes_groups.py	93.81% <97.16%> (-0.36%)	⬇️

... and 1 file with indirect coverage changes