test: Add TCP transport test suite

[Copilot]

TCP transport (state machine, segment parsing, retransmission, window management) had zero test coverage.

Test Coverage (72 tests)

Header Parsing (19 tests)

• Field extraction/writing: ports, sequence/ack numbers, flags, window size
• All flag combinations (SYN/ACK/FIN/RST/PSH/URG/ECE/CWR/NS)
• DataOffset and TryConsume methods
• Endianness conversion validation

Checksums (7 tests)

• IPv4 pseudo-header calculation
• Partial checksum for split buffers
• Validation against known packet captures

State Machine (15 tests)

• All transition paths: CLOSED→LISTEN→SYN_RCVD→ESTABLISHED
• Teardown: FIN_WAIT_1/2, CLOSING, TIME_WAIT, CLOSE_WAIT, LAST_ACK

Options (12 tests)

• MSS, Window Scale, SACK Permitted, Timestamps
• NoOp padding and End-of-Options handling

Edge Cases (19 tests)

• Zero-window probing, RST handling, out-of-order segments
• Sequence number wrap-around, duplicate ACKs
• Min/max header sizes (20/60 bytes)

Pattern

Tests use type-punning for zero-copy header access:

var span = tcpPacket.AsSpan();
ref var header = ref Unsafe.As<byte, TcpHeader>(ref MemoryMarshal.GetReference(span));

// Validate checksum with pseudo-header
var pseudoSum = Checksum.PartialCalculate(ref Unsafe.As<TcpV4PseudoHeader, byte>(ref pseudo), ...);
header.SetChecksum(span, pseudoSum);

Follows existing test patterns from Magma.Internet.Ip.Facts with HexToByteArray for known-good packets.

Original prompt

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This section details on the original issue you should resolve

<issue_title>test: TCP transport</issue_title>
<issue_description>## Problem

Magma.Transport.Tcp has a connection state machine, segment parsing, retransmission logic, and window management - all untested. This is the highest-risk untested area.

Sub-tasks

• Create test/Magma.Transport.Tcp.Facts project, add to solution
• TCP header parsing: construct known-good segments in byte[], verify field extraction
• State machine transitions: CLOSED -> SYN_SENT -> ESTABLISHED -> FIN_WAIT etc.
• Checksum computation: validate against known packet captures
• Edge cases: zero-window probing, RST handling, out-of-order segments

Dependencies

Depends on: nothing (tests existing code)
Unlocks: #105 (UDP datagram), #122 (TCP benchmarks), #131 (Kestrel), establishes test patterns for other projects

Phase

Phase 0 (Foundation) - can start immediately</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes test: TCP transport #103

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