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Add vtcp.Conn: full TCP connection engine with state machine

Phase 2 of the vtcp implementation — the core Conn type that implements
net.Conn with a complete RFC 793 TCP state machine.

Features:
- Active open (Connect) and passive open (AcceptSYN)
- Full state machine: CLOSED → SYN-SENT/SYN-RECEIVED → ESTABLISHED →
  FIN-WAIT-1/2 → TIME-WAIT, CLOSE-WAIT → LAST-ACK, CLOSING
- Data transfer with Read/Write implementing net.Conn
- Send buffer integration with congestion window (RFC 5681 NewReno)
- Receive buffer with out-of-order reassembly
- RTO retransmission with exponential backoff (RFC 6298)
- Fast retransmit on 3 duplicate ACKs
- Keepalive probes (configurable, RFC 1122)
- Read/write deadlines
- Graceful close (FIN) and abort (RST)
- Queue+drain pattern: all packets sent outside mutex, safe for
  synchronous Pipe delivery

Design:
- HandleSegment(seg) returns packets to send (caller sends them)
- SegmentWriter callback for timer-driven sends (RTO, keepalive)
- IP-agnostic: operates on raw TCP segments only
- Thread-safe: concurrent Read/Write/HandleSegment

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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