cleanup: remove mod.rs and other lints

[slawlor]

Also this adjusts the unsafe code build failure such that additional,
non-allowlisted unsafe code will fail the build for safety.

Ractor should have a single unsafe entry in it, specifically for error
handling and auto-conversion.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 80.56%. Comparing base (346f05a) to head (6011044).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ractor_cluster/src/net.rs	0.00%	12 Missing ⚠️
ractor_cluster/src/node.rs	0.00%	4 Missing ⚠️
ractor_cluster/src/net/listener.rs	0.00%	1 Missing ⚠️
ractor_cluster/src/node/node_session.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #347      +/-   ##
==========================================
- Coverage   80.62%   80.56%   -0.06%     
==========================================
  Files          66       66              
  Lines       13247    13256       +9     
==========================================
  Hits        10680    10680              
- Misses       2567     2576       +9     

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