Fix TCP chunk: seqNum/ackNum and Clone

[sirzooro]

Description

Self-contained improvements to the existing chunkTCP struct.

• Add seqNum and ackNum fields (replacing commented-out stubs).
• Fix Clone() to copy flags, seqNum, ackNum, tag, and duplicate — previously these were silently dropped on clone.
• Use the tcp package constant in Network() instead of raw string "tcp".

Split out from #375

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.78%. Comparing base (1bb98ad) to head (f07aab1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #387      +/-   ##
==========================================
+ Coverage   84.63%   84.78%   +0.15%     
==========================================
  Files          38       38              
  Lines        3117     3122       +5     
==========================================
+ Hits         2638     2647       +9     
+ Misses        348      345       -3     
+ Partials      131      130       -1     

Flag	Coverage Δ
go	84.78% <100.00%> (+0.15%)	⬆️

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[sirzooro]

Thanks