perf: Combat perf improvements

[ilaykav]

• Closes # - Perf Improvement
• Tests included or not required because: all tests pass, Added combat benchmark
• Release notes not necessary because: internal perf improvement

Devloped on venv, ran benchamrk locally, main changes:

• Replaced np.dot(x, np.ones(...)) with numpy broadcasting ([:, np.newaxis])
• Extract .values from DataFrames before computation loops
  Benchmarked locally on 50K cells × 500 genes, 2-3 speedup on pp.combat

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.60%. Comparing base (1f9b5ca) to head (c0e36aa).
⚠️ Report is 9 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4070      +/-   ##
==========================================
- Coverage   78.62%   78.60%   -0.02%     
==========================================
  Files         117      118       +1     
  Lines       12727    12753      +26     
==========================================
+ Hits        10006    10024      +18     
- Misses       2721     2729       +8     

Flag	Coverage Δ
hatch-test.low-vers	77.90% <100.00%> (-0.02%)	⬇️
hatch-test.pre	78.48% <100.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
src/scanpy/preprocessing/_combat.py	88.46% <100.00%> (-0.27%)	⬇️

... and 11 files with indirect coverage changes

[ilaykav]

~1.5x-2x times faster on 3K-20K cells, measured locally on my Intel i7-8665U python 3.14