Fix split perf bug and progress bar bug

[mdekstrand]

This fixes a small bug in progress bar display, and dramatically speeds up split performance.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.23%. Comparing base (0e27ced) to head (19f03d5).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/logging/progress/_rich.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #546   +/-   ##
=======================================
  Coverage   90.23%   90.23%           
=======================================
  Files          98       98           
  Lines        5756     5756           
=======================================
  Hits         5194     5194           
  Misses        562      562           

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