Refactor NAT to support TCP, fix typos

[sirzooro]

Refactor and extend the NAT translation layer.

• Fix typos in error variable names: errNATRequriesMapping → errNATRequiresMapping, errMismatchLengthIP (spelling), errNonUDPTranslationNotSupported → errTranslationNotSupported.
• Extract the NAPT outbound logic into a shared translateOutboundNAPT closure (previously duplicated per-protocol).
• Add a tcpPortCounter field.
• Add case tcp: branches in translateOutbound and translateInbound to support TCP NAT/NAPT and 1:1 NAT.
• Update nat_test.go to cover the new TCP NAT cases.

Split out from #375

[codecov]

Codecov Report

❌ Patch coverage is 85.59322% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.62%. Comparing base (a4a04ed) to head (813c2c5).

Files with missing lines	Patch %	Lines
vnet/nat.go	85.59%	10 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
- Coverage   84.72%   84.62%   -0.11%     
==========================================
  Files          38       38              
  Lines        3122     3173      +51     
==========================================
+ Hits         2645     2685      +40     
- Misses        347      354       +7     
- Partials      130      134       +4     

Flag	Coverage Δ
go	84.62% <85.59%> (-0.11%)	⬇️

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[sirzooro]

There is a lot of whitespace changes here. In diff options (gear icon next to "Submit review" button on the top) you can enable split layout and hide whitespaces to see actual changes.

[Copilot]

Pull request overview

This PR refactors the vnet NAT translation logic to share NAPT outbound behavior between protocols and extends NAT/NAPT translation to support TCP alongside UDP, while also cleaning up several error variable typos and expanding test coverage accordingly.

Changes:

• Renames/fixes NAT error variables and clarifies unsupported-protocol handling.
• Extracts shared outbound NAPT mapping logic and adds a separate TCP port counter.
• Adds TCP translation paths for outbound/inbound NAT/NAPT and updates NAT tests to run against both UDP and TCP.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
vnet/nat.go	Refactors outbound NAPT mapping into a shared closure, adds TCP translation support and a TCP port counter, and renames error variables.
vnet/nat_test.go	Refactors NAT tests to be protocol-parameterized (UDP/TCP) and adds TCP coverage across existing NAT behavior scenarios.

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[sirzooro]

fixed

[JoTurk]

i would do this in multiple commits, but since it's vnet ig that's not important.

[JoTurk]

thank you.