Expose batch results as a row-by-row iterator

[mdekstrand]

This updates the batch inference code to expose a generator of individual result rows.

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.52%. Comparing base (a3282e8) to head (48b8bde).

Files with missing lines	Patch %	Lines
src/lenskit/batch/_runner.py	88.88%	2 Missing ⚠️
src/lenskit/batch/_results.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1128      +/-   ##
==========================================
- Coverage   89.53%   89.52%   -0.01%     
==========================================
  Files         256      256              
  Lines       17167    17184      +17     
==========================================
+ Hits        15370    15384      +14     
- Misses       1797     1800       +3     

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