Exercise a CallGraphNode! code path

[wheeheee]

Found N=275 works with just a little bit of brute force. Added it to complex_forward.jl.

Improves codecov.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.22%. Comparing base (54e2152) to head (9dd7cb1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   95.99%   96.22%   +0.23%     
==========================================
  Files           4        4              
  Lines         424      424              
==========================================
+ Hits          407      408       +1     
+ Misses         17       16       -1     

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