Implement TCP connection (tcpConn)

[sirzooro]

Add tcpConn type that implements transport.TCPConn / net.Conn.

• Read / Write with in-order delivery using seqNum/ackNum.
• Close with FIN/RST handling.
• SetDeadline, SetReadDeadline, SetWriteDeadline.
• Full unit tests covering connection lifecycle, data exchange, and teardown.

Split out from #375

[codecov]

Codecov Report

❌ Patch coverage is 72.69373% with 74 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.61%. Comparing base (0ccb39d) to head (7106641).

Files with missing lines	Patch %	Lines
vnet/tcp_conn.go	72.59%	58 Missing and 16 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #390      +/-   ##
==========================================
- Coverage   84.11%   83.61%   -0.51%     
==========================================
  Files          41       41              
  Lines        3356     3612     +256     
==========================================
+ Hits         2823     3020     +197     
- Misses        395      437      +42     
- Partials      138      155      +17     

Flag	Coverage Δ
go	83.61% <72.69%> (-0.51%)	⬇️

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[Copilot]

Pull request overview

This PR introduces an initial vnet.TCPConn implementation intended to satisfy transport.TCPConn / net.Conn semantics inside the vnet simulator, along with unit tests for core I/O and teardown behaviors. It also standardizes timeout errors by switching to a shared errIOTimeout instance.

Changes:

• Implement TCPConn read/write paths with simple seq/ack-based blocking semantics, plus close and deadline handling.
• Add a comprehensive new tcp_conn_test.go suite covering reads, writes, deadlines, close behavior, and RST handling.
• Replace per-call timeout error construction with a reusable errIOTimeout.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
vnet/tcp_conn.go	Adds the TCPConn implementation (I/O, close, deadlines) and inbound segment handling.
vnet/tcp_conn_test.go	Adds unit tests for TCPConn data flow, deadlines, shutdown, and RST behavior.
vnet/errors.go	Introduces errIOTimeout global to replace newTimeoutError.
vnet/conn.go	Updates UDPConn timeout path to use errIOTimeout.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.