Store norm-network messages bra-to-ket and gauge with their Hermitian roots

[mtfishman]

Summary

Stores NormNetwork messages as operators from the ket bond leg to the bra bond leg, the split in which a message is the positive Gram matrix of its environment. There one is the trivial environment, tr is a positive normalization, and the apply_operator gauge is the Hermitian root pair from sqrth_invsqrth_safe applied with apply, with no sign fix-ups. Adds a free-fermion test on a comb tree and a cycle.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.09%. Comparing base (5119963) to head (3a64dfc).

Files with missing lines	Patch %	Lines
src/apply/apply_operators.jl	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
+ Coverage   85.83%   86.09%   +0.25%     
==========================================
  Files          15       15              
  Lines         685      669      -16     
==========================================
- Hits          588      576      -12     
+ Misses         97       93       -4     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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