EXP: debug multisigfile loading

[ctb]

NOTE: PR into #3305

code to support sourmash-bio/sourmash_plugin_branchwater#445

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.49%. Comparing base (26b50f3) to head (63df7a8).

Files with missing lines	Patch %	Lines
src/core/src/manifest.rs	72.72%	6 Missing ⚠️
src/core/src/collection.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3333      +/-   ##
==========================================
- Coverage   86.51%   86.49%   -0.03%     
==========================================
  Files         137      137              
  Lines       16023    16048      +25     
  Branches     2728     2728              
==========================================
+ Hits        13863    13880      +17     
- Misses       1851     1859       +8     
  Partials      309      309              

Flag	Coverage Δ
hypothesis-py	25.40% <ø> (ø)
python	92.37% <ø> (ø)
rust	62.28% <73.07%> (+0.04%)	⬆️

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