perf: add concat_strategy argument to Loader

[ilan-gold]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.03%. Comparing base (bdf6041) to head (0c50ab9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/annbatch/loader.py	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
- Coverage   93.77%   91.03%   -2.74%     
==========================================
  Files          10       10              
  Lines         755      770      +15     
==========================================
- Hits          708      701       -7     
- Misses         47       69      +22     

Files with missing lines	Coverage Δ
src/annbatch/loader.py	89.01% <95.83%> (-4.79%)	⬇️

... and 2 files with indirect coverage changes

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