test: TCP transport

[benaadams]

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