perf: chan's parallel mean-var algorithm for dask

[ilan-gold]

See https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm

Based on a #4118 (comment) with @zboldyga

This has two benefits - it allows us to calculate mean/var in one pass instead of effectively two (square sum and sum squared) and gets rid of a numerical instability issue that @zboldyga found the solution to (see removed comment)

• Closes #
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

❌ Patch coverage is 96.49123% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.66%. Comparing base (d66537f) to head (17f706e).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/get/_aggregated.py	96.49%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4143      +/-   ##
==========================================
+ Coverage   79.59%   79.66%   +0.07%     
==========================================
  Files         120      120              
  Lines       12786    12833      +47     
==========================================
+ Hits        10177    10224      +47     
  Misses       2609     2609              

Flag	Coverage Δ
hatch-test.low-vers	78.62% <29.82%> (-0.22%)	⬇️
hatch-test.pre	79.53% <96.49%> (+0.08%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/get/_aggregated.py	93.72% <96.49%> (+0.42%)	⬆️

... and 1 file with indirect coverage changes