Improve sparse buffer multiplication

[thorek1]

Summary

• add threaded buffer implementation with five-argument scaling
• allow benchmarking with a buffer that stores additional zero entries
• update benchmark output

Testing

• julia -t auto benchmark/sparse_buffer_mult.jl | tee benchmark/sparse_buffer_mult_output.txt (interrupted)

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https://chatgpt.com/codex/tasks/task_e_684b2fd24fd4832fb24bbb8a81955d53

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.79%. Comparing base (39fcd87) to head (a239fcf).

Additional details and impacted files

@@                Coverage Diff                 @@
##           rethink_parser     #118      +/-   ##
==================================================
+ Coverage           87.57%   87.79%   +0.21%     
==================================================
  Files                  18       18              
  Lines               10523    10557      +34     
==================================================
+ Hits                 9216     9269      +53     
+ Misses               1307     1288      -19     

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