refactor(toolkit): toolkit/pcapng.py imports the private _tcp_segment from toolkit/pcap.py #1570

pcapkit/toolkit/pcapng.py:21 imports a private helper from another module: from pcapkit.toolkit.pcap import _tcp_segment. That is the pattern #1519 set out to remove. The import arrived with #1556 (e2be74f71) on 2026-10-10, after #1519's inventory was drawn up, so nothing tracks it. The #1568 cross-review found it.

On main (a3da376dd), _tcp_segment is defined at toolkit/pcap.py:64. It is used at toolkit/pcap.py:273 and :320, and at toolkit/pcapng.py:167 and :216.

Expected: give the helper a public home, chosen by usage under #1519's rulings. #1525's test_start_line precedent made a helper public in the base module that both callers already import. Update both callers, and document the helper under docs/source/.