Fix bug handling subregions in true_excise

[rosteen]

Looking into #1167 led me to realize that the subregion handling in true_excise was actually using the upper and lower bounds from the full region rather than each subregion. This fixes that. I'll add a test tomorrow that would have caught this, since right now it looks like all tests still pass with the change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.89%. Comparing base (173d806) to head (8edb04c).
Report is 37 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1170   +/-   ##
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  Coverage   86.89%   86.89%           
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  Files          63       63           
  Lines        4547     4547           
=======================================
  Hits         3951     3951           
  Misses        596      596           

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[rosteen]

I'm actually going to close this without merging, I didn't realize true_excise is not available to call independently from excise_regions, and the current behavior is as documented. Might be worth a deeper revisit sometime but for now I'll leave it as-is.