update negative interest check to use relative tolerance for log level

[dpaiton]

if above the absolute threshold, log an error.
if also above relative threshold, level is critical; else level is warning.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.91%. Comparing base (464a21f) to head (94c1c53).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/agent0/hyperfuzz/system_fuzz/invariant_checks.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1735      +/-   ##
==========================================
- Coverage   81.10%   80.91%   -0.20%     
==========================================
  Files          96       96              
  Lines        7118     7120       +2     
==========================================
- Hits         5773     5761      -12     
- Misses       1345     1359      +14     

Flag	Coverage Δ
unittests	80.91% <50.00%> (-0.20%)	⬇️

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