test: add missing reassembly engine test coverage

[Zious11]

Summary

Test gaps identified during PR #10 review.

Required Tests

1. SYN+ACK / bidirectional data — Full 3-way handshake, data both directions, verify direction assignment
2. Memcap/max_flows eviction — Tiny limits, exceed them, verify eviction order and CloseReason::MemoryPressure
3. FIN teardown through engine — Full teardown, verify CloseReason::Fin and flow removal
4. Anomaly finding generation — Trigger >50 overlaps, verify findings(). Conflicting overlap finding.
5. max_segments_per_direction limit — Exceed segment limit, verify DepthExceeded

Acceptance Criteria

• All 5 test scenarios passing
• make_tcp_packet helper supports ack parameter